AUTOMOTIVE ELECTRICIANS PORTAL
WIRING-DIAGRAMS

Honda Prelude 2.0 Si 1991 – 1991 ENGINE PERFORMANCE Self Diagnostics

  1. Home
  2. >HONDA
  3. >Honda 1991
  4. >Honda Prelude 2.0 Si 1991
  5. >Honda Prelude 2.0 Si 1991 - DTC - Fuses - Component Locations

Download PDF file

Honda Prelude 2.0 Si 1991 - 1991 ENGINE PERFORMANCE Self-Diagnostics


Honda Prelude 2.0 Si 1991 - INTRODUCTION

If no faults were found while performing BASIC TESTING , proceed with self-diagnostics. If no fault codes or only pass codes are present after entering self-diagnostics, proceed to TESTS W/O CODES article in the ENGINE PERFORMANCE Section for diagnosis by symptom (i.e., ROUGH IDLE, NO START, etc.).


Honda Prelude 2.0 Si 1991 - SELF-DIAGNOSTIC SYSTEM Hard Failures


Hard failures cause CHECK ENGINE light to illuminate and remain on until problem is repaired. If light comes on and remains on (light may flash) during vehicle operation, cause of malfunction must be determined using diagnostic (code) charts. If a sensor fails, control unit will use a substitute value in its calculations to continue engine operation. In this condition, commonly known as limp-in mode, the vehicle runs but driveability will not be optimum.


Honda Prelude 2.0 Si 1991 - Intermittent Failures


Intermittent failures may cause CHECK ENGINE light to flicker or illuminate and go out after the intermittent fault goes away. However, the corresponding trouble code will be retained in ECU memory. If related fault does not reoccur within 50 starter operations, related trouble code will be erased from control unit memory. Intermittent failures may be caused by sensor, connector or wiring related problems. See INTERMITTENTS in TESTS W/O CODES article in the ENGINE PERFORMANCE Section.


Honda Prelude 2.0 Si 1991 - CHECK ENGINE LIGHT


All models are equipped with a CHECK ENGINE light. As a bulb check, light illuminates when ignition is on and engine is not running. CHECK ENGINE light also illuminates when a system failure has been detected and a corresponding trouble code has been set in ECU memory. Not all trouble codes will activate CHECK ENGINE light. If CHECK ENGINE light is on and no codes are flashed by LED on ECU, back-up system is in operation. See TESTS W/O CODES article in the ENGINE PERFORMANCE Section.


Honda Prelude 2.0 Si 1991 - RETRIEVING CODES


On 2.1L, with ignition off, insert jumper wire in service check connector terminal, located near underhood fuse/relay block. See Fig. 1 . On all models, turn ignition on and count flashes of LED inspection light on ECU.

On all models, see TROUBLE CODE IDENTIFICATION CHART and proceed to appropriate code for testing. All voltage tests should be performed with a Digital Volt-Ohmmeter (DVOM) with a minimum 10-megohm input impedance, unless specifically stated differently in testing procedures.

Honda Prelude 2.0 Si 1991 - Component Locations - Locating Service Check Connector 2.1L https://portal-diagnostov.com/license.html
https://portal-diagnostov.com/license.html Automotive Electricians Portal FZCO Automotive Electricians Portal FZCO
Download PDF file
Fig. 1: Honda Prelude 2.0 Si 1991 - Component Locations - Locating Service Check Connector 2.1L


Honda Prelude 2.0 Si 1991 - TROUBLE CODE DEFINITION



Honda Prelude 2.0 Si 1991 TROUBLE CODE IDENTIFICATION CHART

Code (1) System Affected Probable Cause
0 (2) ECU No signal to ECU
1 O2 Sensor A (2.0L) Problem in O2 sensor circuit
1 Heated O2 Sensor (2.1L) Problem in O2 sensor circuit
2 O2 Sensor B (2.0L) Problem in O2 sensor circuit
3 MAP Sensor Electrical problem in MAP sensor
4 Crank Angle Sensor Problem in CRANK circuit of sensor
5 MAP Sensor Mechanical problem in MAP sensor
6 Coolant Temperature (TW) Sensor Open or shorted circuit signal
7 Throttle Angle Sensor Open or shorted signal circuit
8 TDC Sensor Problem in TDC circuit of sensor
9 No. 1 Cylinder Position Sensor Problem in CYL circuit of sensor
10 Intake Air Temperature Open or shorted circuit signal
12 EGR System No EGR action
13 Atmospheric Pressure (PA) Open or shorted circuit signal
14 Electronic Air Control Valve (EACV) Open or shorted EACV circuit
15 Ignition Output Signal Open or shorted signal circuit
16 Fuel Injector Open or shorted signal circuit
17 Vehicle Speed Sensor (VSS) Open or shorted signal circuit
41 O2 Sensor Heater (2.1L) Problem in O2 sensor circuit
43 Fuel Supply System (2.1L) Problem in O2 sensor circuit or fuel system
(1) If codes other than these are indicated, repeat self-diagnosis. If code(s) reappear, substitute with a known good ECU, and recheck code(s). If code(s) clear, replace ECU.
(2) NO CHECK ENGINE LIGHT or CHECK ENGINE LIGHT ON, LED WON'T FLASH CODE for the 2.0L or CHECK ENGINE LIGHT ON, LED WON'T FLASH CODE for the 2.1L.


Honda Prelude 2.0 Si 1991 - CLEARING CODES


To reset ECU, remove CLOCK fuse located in underhood fuse/relay block. Leave fuse out for 10 seconds to reset ECU.


Honda Prelude 2.0 Si 1991 ECU RESET FUSE CHART

Model Fuse Location
Accord BACK-UP (1)
Civic ECU (2)
Prelude CLOCK (1)
(1)
Underhood fuse/relay box.
(2)
Main fuse block.


Honda Prelude 2.0 Si 1991 - ECU LOCATION


ECU is located under carpet in passenger-side footwell.


Honda Prelude 2.0 Si 1991 - DIAGNOSTIC CODES NO CHECK ENGINE LIGHT


  1. Turn ignition on. If oil pressure light is on, go to step 2). If light is off, inspect fuse No. 13. If fuse is okay, repair open circuit in Yellow wire between fuse and combination meter.
  2. Turn ignition off. Connect appropriate ECU Test Harness (07999-PD6000A or 07LAJ-PT3010A ) between ECU and connector. -
    Fig.. Fig. 2 and Fig. 3 . Connect terminal A13 on 2.1L (B6 on 2.0L) to ground. See Fig. 4 and Fig. 5 . Turn ignition on. If CHECK ENGINE light is now on, go to step 3) for 2.1L models or step 4) for 2.0L models. If light is off, replace light bulb or repair open in Green/Red wire between ECU terminal A13 and combination meter.

    Honda Prelude 2.0 Si 1991 - Component Locations - Connecting PGM-FI Test Harness (07999-PD6000A) (2.0L) https://portal-diagnostov.com/license.html
    https://portal-diagnostov.com/license.html Automotive Electricians Portal FZCO Automotive Electricians Portal FZCO
    Get access to all wiring diagrams of the carDownload PDF file

    Fig. 2: Honda Prelude 2.0 Si 1991 - Component Locations - Connecting PGM-FI Test Harness (07999-PD6000A) (2.0L)

    Honda Prelude 2.0 Si 1991 - Component Locations - Connecting ECU Test Harness (07LAJ-PT3010A) (2.1L) https://portal-diagnostov.com/license.html
    https://portal-diagnostov.com/license.html Automotive Electricians Portal FZCO Automotive Electricians Portal FZCO
    Get access to all wiring diagrams of the carDownload PDF file

    Fig. 3: Honda Prelude 2.0 Si 1991 - Component Locations - Connecting ECU Test Harness (07LAJ-PT3010A) (2.1L)

    Honda Prelude 2.0 Si 1991 - Component Locations - Identifying ECU Terminals (2.0L) https://portal-diagnostov.com/license.html
    https://portal-diagnostov.com/license.html Automotive Electricians Portal FZCO Automotive Electricians Portal FZCO
    Get access to all wiring diagrams of the carDownload PDF file

    Fig. 4: Honda Prelude 2.0 Si 1991 - Component Locations - Identifying ECU Terminals (2.0L)

    Honda Prelude 2.0 Si 1991 - Component Locations - Identifying ECU Terminals (2.1L) https://portal-diagnostov.com/license.html
    https://portal-diagnostov.com/license.html Automotive Electricians Portal FZCO Automotive Electricians Portal FZCO
    Get access to all wiring diagrams of the carDownload PDF file

    Fig. 5: Honda Prelude 2.0 Si 1991 - Component Locations - Identifying ECU Terminals (2.1L)

  3. On 2.1L, individually measure voltage between ground and ECU terminals A23, A24, A26 and B2. If less than one volt is present, substitute a known good ECU, and recheck. If condition is corrected, replace original ECU. If more than one volt is present, repair open in wire between ECU and thermostat housing ground having more than one volt.
  4. On 2.0L, individually connect ECU terminal A2 and then terminal A4 to terminal B6. If CHECK ENGINE light fails to illuminate, repair open in Black A2 or A4 wires between ECU and ground.
  5. If CHECK ENGINE light illuminates in step 4), substitute a known good ECU. If condition is corrected, replace original ECU.

Honda Prelude 2.0 Si 1991 - CHECK ENGINE LIGHT ON, LED WON'T FLASH CODE (2.0L)


  1. Turn ignition on. If CHECK ENGINE light stays on after 2 seconds and LED does not blink, start engine.
  2. If CHECK ENGINE light stays on, go to step 3). If CHECK ENGINE light does not stay on, problem is intermittent. Test drive vehicle, and check for loose wires and connections. Check fuse No. 12 on fuse/relay block behind left side of dash or ECU fuse in underhood fuse/relay block.
  3. If LED flashes codes, repair trouble code(s). If LED does not flash any code(s) but engine starts, go to step 8). If engine does not start, remove and inspect ECU (10A) fuse in underhood fuse/relay block. Replace if necessary. If fuse is okay, go to step 4).
  4. Inspect fuse No. 12 (10A) in fuse/relay block behind left side of dash. Replace if necessary. If fuse is okay, turn ignition on. Individually disconnect connectors from MAP, TPS and EGR lift sensors while observing CHECK ENGINE light. If CHECK engine light goes off, replace sensor causing light to go off when disconnected.
  5. If CHECK ENGINE light remains on, disconnect Atmospheric Pressure (PA) sensor connector. If LED flashes Code 13, replace PA sensor. If LED does not flash Code 13, reconnect PA sensor connector. Turn ignition off. Connect PGM-FI test harness connector. Disconnect connector "C" from ECU (not main harness).
  6. Turn ignition on. Measure voltage between terminals A18 (-) and A13 (+) and between terminals A18 (-) and A15 (+). If battery voltage is not available, go to step 7). If battery voltage is available, substitute a known good ECU, and retest. If trouble goes away, replace ECU.
  7. If voltage is not available between terminals A13, A15 and A18, repair open in Yellow/Black wire between ECU terminals A13, A15 and main relay. Check main relay and connections.
  8. Turn ignition off. Disconnect ECU connector "B". Turn ignition on. If CHECK ENGINE light illuminates, repair short to ground in Yellow/Red wire between ECU terminal B6 and CHECK ENGINE light. If CHECK ENGINE light does not illuminate, substitute a known good ECU, and recheck. If symptoms go away, replace ECU.

    NOTE: Ensure service check connector is NOT jumpered or CHECK ENGINE light will stay on.


Honda Prelude 2.0 Si 1991 - CHECK ENGINE LIGHT ON, LED WON'T FLASH CODE (2.1L)


  1. Turn ignition on. If CHECK ENGINE light stays on after 2 seconds, go to step 2). If CHECK ENGINE light turns off, problem is intermittent. Test drive vehicle. Check for loose wires and connections at thermostat housing and main relay connector. Check ECU fuse at underhood fuse/relay block and fuse No. 12 at fuse/relay block, behind left side of dash.
  2. Check if LED is flashing code(s). If LED is flashing codes, repair trouble codes. If LED won't flash code(s), try starting engine. If engine does not start, go to next step. If engine starts, go to step 8).
  3. Remove and inspect ECU (10A) fuse in underhood fuse/relay block. Replace as necessary. If fuse is okay, inspect fuse No. 12 (10A) at fuse/relay block, behind left side of dash. Replace as necessary. If fuse is okay, go to next step.
  4. Turn ignition on. Individually disconnect 3-wire connector at MAP, TPS and EGR valve lift sensors while observing CHECK ENGINE light. If CHECK engine light goes off, replace sensor causing light to go off when disconnected. If CHECK ENGINE light stays on, go to next step.
  5. Turn ignition off. Connect ECU Test Harness (07999-PD6000A ). Disconnect ECU connector "D" from ECU, not main wire harness. Using an ohmmeter, check continuity between body ground and terminals D19 and D20. If continuity does not exist, go to step 6). If continuity exists, check short to ground in Red/White wire or Yellow/White wire between ECU terminals D19 and D20 and MAP, TPS and EGR valve lift sensors.
  6. Reconnect all connectors. Reconnect connector "D" to ECU. Turn ignition on. Using a voltmeter, measure voltage between ECU terminals A26 (-) and B1 (+) and between terminals A26 (-) and A25 (+).
  7. If battery voltage is present at these terminals, substitute a known good ECU, and recheck. If symptom goes away, replace original ECU. If battery voltage is not present, repair open in Yellow/Black wire between ECU terminals A25 and B1 and main relay. Check main relay and wiring connectors at main relay.
  8. Using a voltmeter, measure voltage between Brown (+) and Green/White (-) wires on service check connector. If 5 volts are not present, repair short to ground in Brown wire between ECU terminal D4 and service check connector. If 5 volts are present, go to next step.
  9. Turn ignition off. Disconnect ECU "A" connector. Turn ignition on. If CHECK ENGINE light illuminates, repair short to ground in Yellow/Red wire between ECU terminal A13 and CHECK ENGINE light. If CHECK ENGINE light does not illuminate, go to next step.
  10. Substitute a known good ECU, and recheck. If CHECK ENGINE light illuminates or symptom goes away, replace original ECU.

Honda Prelude 2.0 Si 1991 - CODE 1 - O2 SENSOR (2.1L)


  1. Turn ignition off. Reset ECU by removing CLOCK fuse in underhood fuse/relay block for 10 seconds. Start engine, and warm to normal operating temperature (cooling fan comes on).
  2. Road test vehicle. With transmission in 2nd gear, accelerate for at least 5 seconds at wide open throttle, and then decelerate for at least 5 seconds with throttle completely closed. Verify if CHECK ENGINE light is on and if LED is flashing Code 1. If LED is flashing Code 1, perform test in CODE 43, FUEL SUPPLY SYSTEM . If LED is not flashing Code 1, problem is intermittent. Check for poor connections and loose wires at ground connector near right shock tower, at O2 sensor and at ECU.

Honda Prelude 2.0 Si 1991 - CODE 1 &/OR CODE 2 - O2 SENSOR (2.0)


NOTE: 2.0L is equipped with 2 O2 sensors. Code 1 indicates problem with O2 sensor "A" and Code 2 indicates problem with O2 sensor "B".

  1. Turn ignition off. Remove CLOCK fuse in underhood fuse/relay block for 10 seconds to reset ECU. Inspect fuel pressure regulator. See SYSTEM/COMPONENT TESTS article in the ENGINE PERFORMANCE Section. Repair or replace as necessary. Warm engine to normal operating temperature (cooling fan comes on).
  2. Hold engine speed at 1500 RPM for 15 minutes. DO NOT close throttle completely. If CHECK ENGINE light comes on and LED indicates Code 1 or 2, go to step 3). If LED does not indicate Code 1 or 2, failure is intermittent. Check for loose wires and connections at ground (right shock tower), O2 sensor and ECU.
  3. Disconnect O2 sensor connector. Connect voltmeter between O2 sensor connector and ground. Warm engine to normal engine temperature again. While watching voltmeter, open throttle wide open and then close throttle. If voltage is greater than .6 volt at wide open throttle and below .4 volt when throttle is quickly released, go to next step. If voltage is not as specified, replace O2 sensor.
  4. Turn ignition and engine off. Connect PGM-FI Test Harness between ECU and connector. Connect voltmeter between ECU harness terminals A18 (+) and C16 (-) for O2 sensor "A". Connect voltmeter between ECU harness terminals A18 (+) and B11 (-) for O2 sensor "B". Restart and warm engine to normal operating temperature. While reading voltmeter, open throttle wide open then quickly close throttle.
  5. If voltage is greater than .6 volt at wide open throttle and below .4 volt when throttle is quickly released, substitute a known good ECU, and recheck. If symptoms go away, replace original ECU. If voltage is not as specified, repair open or short in O2 sensor wire (White wire for O2 sensor "A" and Red/Blue wire for O2 sensor "B").

Honda Prelude 2.0 Si 1991 - CODE 3 - MAP SENSOR


  1. Turn ignition off. Remove appropriate fuse for 10 seconds to reset ECU. See CLEARING CODES . Start engine, and warm to normal operating temperature (cooling fan comes on). Verify if CHECK ENGINE light is on and if LED is flashing Code 3. If Code 3 is present, go to next step. If Code 3 is not present, problem is intermittent. Check connectors at MAP sensor and at ECU.
  2. Turn ignition off. Disconnect 3-wire connector from MAP sensor. Turn ignition on. Measure voltage between Red/White (+) wire and ground. If reading is about 5 volts, go to step 3). If reading is not about 5 volts, go to step 7).
  3. Measure voltage between Red/White (+) and Blue/White (-) wires. If about 5 volts are present, go to step 4). If about 5 volts are not present, repair open in Blue/White wire between ECU terminal D21 (C14 on 2.0L) and MAP sensor. If wire is okay, substitute a known good ECU. If condition is corrected, replace original ECU.
  4. Measure voltage between White/Blue (+) and Blue/White (-) wires. If voltage is about 5 volts, go to step 5). If about 5 volts are not present, repair open in White/Blue wire between ECU terminal D17 (C11 on 2.0L) and MAP sensor. If wire is okay, substitute a known good ECU. If condition is corrected, replace original ECU.
  5. Turn ignition off. Reconnect 3-wire connector to MAP sensor. Connect appropriate ECU test harness between ECU and ECU harness. See Fig. 2 and Fig. 3 . Turn ignition on. Measure voltage between terminals D17 and D21 (C11 and C14 on 2.0L). See Fig. 4 and Fig. 5 .
  6. If reading is not about 3 volts, replace MAP sensor. If about 3 volts are present, substitute a known good ECU, and recheck. If condition is corrected, replace original ECU.
  7. Connect ECU Test Harness to ECU only. Measure voltage between ECU terminals D19 and D21 (C15 and C14 on 2.0L). If about 5 volts are present, repair open in Red/White wire between ECU terminal D19 (C15 on 2.0L) and MAP sensor. If about 5 volts are not present, substitute a known good ECU, and retest. If condition is corrected, replace original ECU.

Honda Prelude 2.0 Si 1991 - CODE 4 - CRANK ANGLE SENSOR


  1. Turn ignition off. Remove CLOCK fuse for 10 seconds to reset ECU. Start engine. Verify if CHECK ENGINE light is on and if LED is flashing Code 4. If LED is flashing Code 4, go to next step. If LED is not flashing Code 4, problem is intermittent. Check connectors at distributor and related circuits.
  2. Turn ignition off. Disconnect 4-wire TDC/CRANK sensor connector nearest sensor. Measure resistance across terminals "C" and "D" at sensor. See Fig. 6 . If reading is not 700-1000 ohms, replace pulse generator assembly. If reading is within specification, proceed to next step.
  3. Check continuity between ground and sensor terminals "C" and "D" individually. If continuity exists on either terminal, sensor is shorted to ground. Replace pulse generator assembly. If no continuity exists, go to next step.
  4. Reconnect sensor connector. Connect appropriate ECU Test Harness (07999-PD6000A or 07LAJ-PT3010A ) to main harness side only. See Fig. 2 and Fig. 3 . Check resistance between connector terminals B10 and B12 (B15 and B16 on 2.1L). See Fig. 4 and Fig. 5 . Resistance should be 700-1000 ohms.
  5. If resistance is not within specification, repair open in Blue/Yellow and/or Blue/Green wire. If resistance is within specification, check for continuity from terminal B10 (B15 on 2.1L) to ground.
  6. If continuity exists, repair short in Blue/Green wire between ECU terminal B10 (B15 on 2.1L) and distributor. If continuity does not exist, substitute a known good ECU, and recheck. If condition is corrected, replace original ECU.

    Honda Prelude 2.0 Si 1991 - Component Locations - TDC/CRANK Sensor Connector Terminal ID https://portal-diagnostov.com/license.html
    https://portal-diagnostov.com/license.html Automotive Electricians Portal FZCO Automotive Electricians Portal FZCO
    Get access to all wiring diagrams of the carDownload PDF file

    Fig. 6: Honda Prelude 2.0 Si 1991 - Component Locations - TDC/CRANK Sensor Connector Terminal ID


Honda Prelude 2.0 Si 1991 - CODE 5 - MAP SENSOR


  1. Turn ignition off. Remove appropriate fuse for 10 seconds to reset ECU. See CLEARING CODES . Start engine. Verify if CHECK ENGINE light is on and LED is flashing Code 5. If Code 5 is present, go to next step. If Code 5 is not present, problem is intermittent. Check vacuum hoses, pipes and connections. Ensure all connectors are secure.
  2. Turn ignition off. Using a "T" fitting, connect a hand-held vacuum pump/gauge into vacuum hose between throttle body and MAP sensor (vacuum hose No. 21). Apply vacuum. If vacuum does not hold, attach vacuum pump/gauge directly to MAP sensor. Apply vacuum. If vacuum does not hold, replace MAP sensor. If vacuum holds, check MAP sensor vacuum hose for cracks, splits and looseness. Repair as necessary.
  3. Check vacuum supply to MAP sensor with engine running. If vacuum is not present, repair hose or clear restriction from throttle body. If vacuum supply is okay, turn ignition off.
  4. Connect appropriate ECU Test Harness (07LAJ-PT3010A or 07999-PD6000A ) between ECU and ECU connector. See Fig. 2 and Fig. 3 . Measure voltage between terminal D17 (C11 on 2.0L) and terminal D21 (C14 on 2.0L). See Fig. 4 and Fig. 5 . If voltage reading is about 3 volts, go to next step. If voltage is not about 3 volts, replace MAP sensor.
  5. Start engine, and let idle. If voltage is not about 1 volt, replace MAP sensor. If voltage reading is about 1 volt, substitute a known good ECU. If condition is corrected, replace original ECU.

Honda Prelude 2.0 Si 1991 - CODE 6 - COOLANT TEMPERATURE (TW) SENSOR


  1. Turn ignition off. Remove appropriate fuse for 10 seconds to reset ECU. See CLEARING CODES . Start engine. Verify if CHECK ENGINE light is on and if LED is flashing Code 6. If Code 6 is present, go to next step. If Code 6 is not present, problem is intermittent. Check connectors at TW sensor and related circuits.
  2. Start engine, and warm to normal operating temperature (cooling fan comes on). Turn ignition off. Disconnect 2-wire coolant sensor connector. Measure resistance across sensor terminals.
  3. If resistance is not 200-400 ohms, replace TW sensor. If resistance is within specification, turn ignition on. Measure voltage between ground and Yellow/Green wire of TW sensor harness. If reading is about 5 volts, go to step 4). If reading is not about 5 volts, go to step 5) for A/T models or step 6) for M/T models.
  4. Measure voltage between Yellow/Green (+) and Green/White (-) wires. If reading is about 5 volts, substitute a known good ECU, and recheck. If condition is corrected, replace original ECU. If voltage is not as specified, repair open in Green/White wire between ECU terminal D22 (C12 on 2.0L) and TW sensor.
  5. Disconnect 22-wire connector (18-wire connector on 2.0L) from A/T control unit. If voltage is now about 5 volts, replace A/T control unit. If reading is not about 5 volts, repair open or short in Yellow/Green wire between ECU terminal D13 (C6 on 2.0L), A/T control unit and TW sensor. If wire is okay, substitute a known good ECU, and recheck. If condition is corrected, replace original ECU.
  6. Repair open or short in Yellow/Green wire between ECU terminal D13 (C6 on 2.0L) and TW sensor. If wire is okay, substitute a known good ECU, and recheck. If condition is corrected, replace original ECU.

Honda Prelude 2.0 Si 1991 - CODE 7 - THROTTLE ANGLE SENSOR


  1. Turn ignition off. Remove appropriate fuse for 10 seconds to reset ECU. See CLEARING CODES . Start engine. Verify if CHECK ENGINE light is on and if LED is flashing Code 7. If Code 7 is present, go to next step. If Code 7 is not present, problem is intermittent. Check connectors at throttle angle sensor and related circuits.
  2. Turn ignition off. Disconnect 3-wire connector at throttle angle sensor. Turn ignition on. Measure voltage between Yellow/White (+) and Green/White (-) wires. If voltage is not about 5 volts, go to step 6). If voltage is about 5 volts, turn ignition off. Reconnect sensor connector. Connect ECU Test Harness between ECU and ECU connector. See Fig. 2 and Fig. 3 .
  3. Turn ignition on. Measure voltage between positive terminal D11 (C7 on 2.0L) and negative terminal D22 (C12 on 2.0L). See Fig. 4 and Fig. 5 . Voltage should be .5 volt at closed throttle and about 4.5 volts at wide open throttle. Transition between lower and upper voltage specification as throttle is depressed should be smooth.
  4. If throttle sensor responds as indicated, substitute a known good ECU. If condition is corrected, replace original ECU. If sensor does not respond as indicated in step 3), replace throttle sensor or repair open or short in Red/Black wire between ECU terminal D11 (C7 on 2.0L) and throttle sensor. On A/T models, go to step 5).
  5. Disconnect 22-wire connector (18-wire connector on 2.0L) from A/T control unit. If throttle sensor now responds as indicated, replace A/T control unit. If throttle sensor does not respond as indicated, check for open or short in Red/Black wire between ECU terminal D11, throttle angle sensor and A/T control unit.
  6. If voltage reading between Yellow/White (+) and Green/White (-) wires in step 2) is not about 5 volts, measure voltage between ground and Yellow/White wire terminal. If voltage is now about 5 volts, repair open in Green/White wire between ECU terminal D22 (C12 on 2.0L) and throttle sensor. If reading is not about 5 volts, turn ignition off.
  7. Connect ECU Test Harness between ECU and ECU connector. Turn ignition on. Measure voltage between terminal D20 (C13 on 2.0L) and terminal D22 (C12 on 2.0L). If voltage is about 5 volts, repair open Yellow/White wire between ECU terminal D20 (C13 on 2.0L) and throttle sensor. If reading is not about 5 volts, substitute a known good ECU. If condition is corrected, replace original ECU.

Honda Prelude 2.0 Si 1991 - CODE 8 - TDC SENSOR


  1. Turn ignition off. Remove CLOCK fuse for 10 seconds to reset ECU. Start engine, and observe CHECK ENGINE light. If light is off, problem is intermittent. Test drive. and check again.
  2. If light is on, turn ignition off. Disconnect 4-wire sensor connector at distributor. Measure resistance between Orange/Blue and White/Blue wires. If reading is not 700-1000 ohms, replace pulse generator assembly. If reading is within specification, proceed to next step.
  3. Check continuity between ground and Orange/Blue wire terminal. Check continuity between ground and White/Blue wire terminal. If continuity exists, sensor is shorted to ground. Replace pulse generator assembly. If no continuity exists, proceed to next step.
  4. Reconnect sensor connector. Attach appropriate ECU Test Harness (07LAJ-PT3010A or 07999-PD6000A ) to main wire harness side only. See Fig. 2 and Fig. 3 . Using an ohmmeter, check resistance between connector C3 terminals and C4 (B13 and B14 on 2.1L). See Fig. 4 and Fig. 5 . Resistance should be 700-1000 ohms.
  5. If resistance is not within specification, repair open or short in Orange/Blue and/or White/Blue wire. If resistance is within specification, check for continuity from terminal C3 (B13 on 2.1L) to ground.
  6. If continuity exists, repair short in Orange/Blue wire between ECU terminal C3 (B13 on 2.1L) and sensor connector. If continuity does not exist, substitute a known good ECU. If condition is corrected, replace original ECU.

Honda Prelude 2.0 Si 1991 - CODE 9 - CYL SENSOR


  1. Turn ignition off. Remove CLOCK fuse for 10 seconds to reset ECU. Start engine. Verify if CHECK ENGINE light is on and if LED is flashing Code 9. If LED is flashing Code 9, go to next step. If LED is not flashing Code 9, problem is intermittent. Check connectors at distributor and related circuits.
  2. Turn ignition off. Disconnect 2-wire sensor connector. Measure resistance across sensor terminals. If reading is not 700-1000 ohms, replace CYL sensor assembly. If reading is within specification, proceed to next step.
  3. Check continuity between ground and each sensor terminal. If continuity exists, replace CYL sensor assembly. If no continuity exists, proceed to next step.
  4. Reconnect sensor connector. Attach ECU Test Harness (07LAJ-PT3010A ) to main wire harness side only. Check resistance between connector terminals C1 and C2 (B11 and B12 on 2.1L). Resistance should be 700-1000 ohms.
  5. If resistance is not within specification, repair open or short in Orange and/or White wires. If resistance is within specification, check continuity between terminal C1 (B11 on 2.1L) and ground. If continuity is not present, repair open in Orange wire between ECU terminal C1 (B11 on 2.1L) and CYL sensor connector. If resistance is within specification, substitute a known good ECU, and retest. If condition is corrected, replace original ECU.

Honda Prelude 2.0 Si 1991 - CODE 10 - AIR TEMPERATURE (TA) SENSOR


  1. Turn ignition off. Remove appropriate fuse for 10 seconds to reset ECU. See CLEARING CODES . Start engine. Verify if CHECK ENGINE light is on and if LED is flashing Code 10. If Code 10 is present, go to next step. If Code 10 is not present, problem is intermittent. Check connectors at TA sensor and related circuits.
  2. Turn ignition off. Disconnect TA sensor connector, and measure resistance across TA sensor terminals. If resistance reading is not 1000-4000 ohms, replace TA sensor. If resistance reading is as specified, disconnect ohmmeter, and turn ignition on. Measure voltage between ground and Red/Yellow wire terminal. If voltage is about 5 volts, go to step 3). If voltage is not about 5 volts, go to step 4).
  3. Measure voltage between Red/White (+) and Green/White (-) wires. If voltage is not about 5 volts, repair open circuit in Green/White wire between ECU terminal D22 (C12 on 2.0L) and TA sensor. If voltage is about 5 volts, substitute a known good ECU, and recheck. If condition is corrected, replace original ECU.
  4. Repair open or short in Red/Yellow wire between ECU terminal D15 (C5 on 2.0L) and TA sensor. If wire is okay, substitute a known good ECU, and recheck. If condition is corrected, replace original ECU.

Honda Prelude 2.0 Si 1991 - CODE 12 - EGR SYSTEM


  1. Turn ignition off. Remove appropriate fuse for 10 seconds to reset ECU. See CLEARING CODES . Start engine, and warm to normal operating temperature. Road test vehicle for about 10 minutes. Try to keep engine speed in 1700-2500 RPM range. Verify if CHECK ENGINE light is on and if LED is flashing Code 12. If Code 12 is present, go to next step. If Code 12 is not present, problem is intermittent. Check connectors at EGR valve and related circuits.
  2. With engine at idle, disconnect vacuum hose No. 16 from EGR valve. Using a hand-held vacuum pump/gauge, test for vacuum. If vacuum is present, go to next step. If vacuum is not present, go to step 5).
  3. Turn ignition off, and disconnect 4-wire connector from vacuum control box. Start engine, and again check for vacuum. If vacuum is present, check vacuum hose routing for entire EGR system. If hose routing is okay, replace EGR solenoid control valve.
  4. If vacuum is not present, check for short to ground in Red wire between EGR solenoid control valve and ECU terminal A11 (B5 on 2.0L). If wire is okay, substitute a known good ECU, and recheck. If condition is corrected, replace original ECU.
  5. Move vacuum pump to EGR valve, and apply 8 in. Hg to EGR valve. If vacuum does not hold and engine does not stall, replace faulty EGR valve or clean blocked EGR passages.
  6. If vacuum holds and engine stalls in step 5), disconnect 4-wire connector from control box. Measure voltage between Black/Yellow (+) terminal on main wire harness and ground. Battery voltage should be present. If battery voltage is not present, repair open in Black/Yellow wire between solenoid valve and fuse No. 12. If battery voltage is present, go to next step.
  7. Reconnect vacuum pump/gauge to EGR vacuum hose No. 16. Start engine, and let idle. Supply battery voltage to Black/Yellow wire terminal of box, and ground to Red wire terminal of box. Reading at EGR valve hose should be about 8 in. Hg after one second. If reading is okay, go to step 10). If reading is not okay, go to step 8).
  8. Turn ignition off. Inspect hoses No. 16 and 24 for leaks, restrictions and misrouting. If problem is found, repair as necessary. If hoses are okay, disconnect lower hose on EGR control solenoid valve, and connect a vacuum gauge.
  9. Start engine, and allow to idle. If 6-10 in. Hg is not present, replace Constant Vacuum Control (CVC) Valve. If 6-10 in. Hg is present, replace EGR control solenoid valve.
  10. If gauge indicates 8 in. Hg in step 7), turn ignition off, and reconnect 4-pin connector. Connect Test Harness (07GMJ-ML80100 - 2.0L, 07LAJ-ML80100 - 2.1L) to EGR valve lift sensor harness. Turn ignition on. Measure voltage between Red (+) and Green (-) wire terminals. If about 5 volts is not present, repair open in Yellow/White wire between ECU terminal D20 (C13 on 2.0L) and EGR valve or Green/White wire between ECU terminal D22 (C12 on 2.0L) and EGR valve.
  11. If 5 volts is present, measure voltage between White (+) and Green (-) wire terminals. If approximately 1.2 volts are not present, replace EGR valve. Repair short in White/Black or Yellow wire between ECU terminal D12 (C8 on 2.0L) and EGR valve lift sensor. If wire is okay, substitute a known good ECU. If proper voltage is present, replace original ECU.
  12. If 1.2 volts are present, observe voltmeter and slowly apply a continuous 8 in. Hg to valve. Repeat several times, fully releasing vacuum between checks. If maximum voltage is not 4 volts, replace EGR valve. If maximum voltage is 4 volts, ensure voltage consistently increases and decreases as vacuum increases and decreases.
  13. If voltage does not increase and decrease with vacuum, replace EGR valve. If voltage increases and decreases with vacuum, reconnect hose No. 16 to EGR valve. Turn ignition off. Connect ECU Test Harness to ECU. See Fig. 2 and Fig. 3 . Start engine, and allow to idle. Measure voltage between ECU positive terminal D12 (C8 on 2.0L) and negative terminal D22 (C12 on 2.0L). See Fig. 4 and Fig. 5 . If approximately 1.2 volts are not present, repair open in White/Black or Yellow wire between ECU terminal D12 (C8 on 2.0L) and EGR valve lift sensor.
  14. If 1.2 volts are present, connect jumper between terminals A11 and A26 (A10 and A18 on 2.0L). If engine does not stall, repair open in Pink wire between ECU terminal A11 (A10 on 2.0L) and EGR control solenoid valve. If engine stalls, substitute a known good ECU. If condition goes away, replace original ECU.

Honda Prelude 2.0 Si 1991 - CODE 13 - ATMOSPHERIC PRESSURE (PA) SENSOR 2.1L


  1. Turn ignition off. Remove appropriate fuse for 10 seconds to reset ECU. See CLEARING CODES .
  2. Turn ignition on. Verify if CHECK ENGINE light is on and if LED is flashing Code 13. If Code 13 is present, go to next step. If Code 13 is not present, problem is intermittent. Test drive may be necessary.
  3. Turn ignition off. Substitute a known good ECU, and recheck. If condition is corrected, replace original ECU.

Honda Prelude 2.0 Si 1991 - 2.0L


  1. Turn ignition off. Remove CLOCK fuse for 10 seconds to reset ECU. Turn ignition on. Verify if CHECK ENGINE light is on and if LED is flashing Code 13. If LED is flashing Code 13, go to next step. If LED is not flashing Code 13, problem is intermittent. Check connectors at PA sensor and related circuits.
  2. Turn ignition off. Connect ECU Test Harness between ECU and ECU connector. See Fig. 2 . Turn ignition on. Measure voltage between terminals C13 (+) and C12 (-). See Fig. 4 . If about 5 volts are present, go to next step. If about 5 volts are not present, substitute a known good ECU, and recheck. If condition is corrected, replace original ECU.
  3. Measure voltage between terminals C9 (+) and C12 (-). If about 3 volts are present, substitute a known good ECU, and recheck. If condition is corrected, replace original ECU. If about 3 volts are not present, disconnect main wire harness from PA sensor.
  4. Measure voltage between ECU terminals C9 (+) and C12 (-). If about 5 volts are not present, repair short in Red wire between ECU terminal C9 and PA sensor. If about 5 volts are present, measure voltage between White/Yellow (+) and Green/White (-) terminals.
  5. If about 5 volts are present, go to step 6). If about 5 volts are not present, measure voltage between White/Yellow terminal and body ground. If about 5 volts are present, repair open in Green/White wire between ECU terminal C12 and PA sensor. If about 5 volts are not present, repair open in White/Yellow wire between ECU terminal C13 and PA sensor.
  6. Measure voltage between Red (+) and Green/White (-) terminals. If about 5 volts are present, replace PA sensor. If about 5 volts are not present, repair open in Red wire between ECU terminal C9 and PA sensor.

Honda Prelude 2.0 Si 1991 - CODE 14 - ELECTRONIC AIR CONTROL VALVE (EACV)


  1. Turn ignition off. Remove appropriate fuse for 10 seconds to reset ECU. See CLEARING CODES . Start engine. Verify if CHECK ENGINE light is on and if LED is flashing Code 14. If Code 14 is present, go to next step. If Code 14 is not present, problem is intermittent. Check connectors at EACV and related circuits.
  2. Turn ignition off. Disconnect EACV connector. Check resistance between both connector terminals on EACV.
  3. Resistance should be 8-15 ohms. If resistance is not within specification, replace EACV. If resistance is within specification, proceed to next step.
  4. Check for continuity to ground on each EACV connector terminal. If continuity exists, replace EACV. If no continuity exists, turn ignition on. Measure voltage between positive Yellow/Black and negative Blue/Red terminals.
  5. Check for battery voltage. If no battery voltage exists, proceed to step 6). If battery voltage exists, disconnect connector from ECU, and again check for battery voltage. If battery voltage still does not exist, substitute a known good ECU. If condition is corrected, replace original ECU. If battery voltage exists, repair short in Blue/Red wire between ECU terminal A9 (A11 on 2.0L) and EACV.
  6. If battery voltage is not present in step 5), measure voltage between ground and Yellow/Black wire terminal. If battery voltage is now present, proceed to step 7). If battery voltage still does not exist, repair open Yellow/Black wire between main relay and EACV.
  7. If battery voltage exists, turn ignition off. Reconnect EACV connector. Connect ECU Test Harness connector "A" to main wire harness side only. Turn ignition on. Measure voltage between connector A9 (+) (A11 on 2.0L) and A23 (-) (A18 on 2.0L). If voltage does not exist, repair open in Black/Blue wire between ECU terminal A9 (A11 on 2.0L) and EACV.
  8. If battery voltage exists, connect and disconnect terminal A9 (A11 on 2.0L) and A23 (A18 on 2.0L). If EACV does not click when connector is connected and disconnected, replace EACV. If EACV clicks, substitute a known good ECU. If condition is corrected, replace original ECU.

Honda Prelude 2.0 Si 1991 - CODE 15 - IGNITION OUTPUT SIGNAL


NOTE: If engine will not start, continue cranking for 20 seconds to reproduce trouble code.

  1. Turn ignition off. Remove appropriate fuse for 10 seconds to reset ECU. See CLEARING CODES . Start engine. Verify if CHECK ENGINE light is on and if LED is flashing Code 15. If Code 15 is present, go to next step. If Code 15 is not present, problem is intermittent. Check connector at distributor and related circuits.
  2. Turn ignition off. Disconnect 6-wire connector at ignitor. Turn ignition on. Measure voltage between ground and Black/Yellow wire terminal on wire harness side. If battery voltage is not present, repair open Black/Yellow wire between connector and ignition switch.
  3. If battery voltage is present, turn ignition off. Reconnect 2-wire or 6-wire connector. Connect ECU Test Harness between ECU and ECU connector. See Fig. 2 and Fig. 3 . Turn ignition on. Individually measure voltage between negative terminal A26 (A18 on 2.0L) and positive terminals A21 and A22 (B15 and B17 on 2.0L). See Fig. 4 and Fig. 5 .

    NOTE: If wire between ignitor and ECU is shorted, ignitor may be damaged.

  4. On 2.1L, if approximately 10 volts is present, substitute a known good ECU, and retest. If condition is corrected, replace original ECU. If specified voltage is not present, check for short or open in White wire between distributor and ECU terminals A21 and A22. If wires are okay, replace ignitor unit.
  5. On 2.0L, if battery voltage is present, substitute a known good ECU, and retest. If condition is corrected, replace original ECU. If battery voltage is not present, turn ignition off.
  6. Disconnect 6-wire ignitor connector from ECU side only. Check for continuity between White wires and ECU terminals B15 and B17 and ignitor. If continuity does not exist, repair as necessary.
  7. If continuity exists, check for continuity between ground and White wires of 6-wire connector. If continuity exists, repair short to ground. If continuity does not exist, replace ignitor unit.

Honda Prelude 2.0 Si 1991 - CODE 16 - FUEL INJECTOR CIRCUIT


  1. Turn ignition off. Remove appropriate fuse for 10 seconds to reset ECU. See CLEARING CODES . Attempt to start engine. If engine does not start, go to step 3). Verify if CHECK ENGINE light is on and if LED is flashing Code 16. If Code 16 is present, go to next step. If Code 16 is not flashing, problem is intermittent. Check connectors at injectors, injector resistor and related circuits.

    NOTE: On models with 2.0L engine, it may be necessary to crank engine for a minimum of 15 seconds to reset trouble code.

  2. Using a stethoscope, check for clicking sound at each injector while engine is idling. If all injectors are clicking, substitute a known good ECU. If condition is corrected, replace original ECU.
  3. Turn ignition off. If engine does not start, check resistance of all injectors. If engine starts, check resistance of injectors not clicking. If resistance value is not 1.5-2.5 ohms, replace injector(s). If resistance values are within specification, turn ignition on. Measure voltage between ground and injector terminal Red/Black wire.
  4. If battery voltage is not present, go to step 6). If battery voltage is present, reconnect injector connector. Turn ignition off. Connect ECU Test Harness between ECU and ECU connector. See Fig. 2 and Fig. 3 . Turn ignition on. Measure voltage between negative terminal A23 (A18 on 2.0L) and following positive terminals. - Fig. 4 and Fig. 5 .
    • Injector No. 1 - A1
    • Injector No. 2 - A3
    • Injector No. 3 - A5
    • Injector No. 4 - A7
  5. If battery voltage is not present, repair open in wire between ECU terminals A1 (injector No. 1), A3 (injector No. 2), A5 (injector No. 3) or A7 (injector No. 4) and injector. If battery voltage is present, substitute a known good ECU. If condition is corrected, replace original ECU.
  6. Turn ignition off. Disconnect injector resistor 6-wire connector. Turn ignition on. Measure voltage between ground and Yellow/Black wire terminal. If battery voltage is present, replace injector resistor or repair open in Red/Black wire between injector connector and main relay. If wire is good, replace injector resistor. If battery voltage is not present, repair open in Yellow/Black wire between injector resistor and main relay.

Honda Prelude 2.0 Si 1991 - CODE 17 - VEHICLE SPEED SENSOR (VSS)


  1. Turn ignition off. Remove appropriate fuse for 10 seconds to reset ECU. See CLEARING CODES . Road test vehicle. With transmission in 2nd gear, accelerate to 3500 RPM and then decelerate to 1500 RPM with throttle fully closed. Verify if CHECK ENGINE light is on and if LED is flashing Code 17. If Code 17 is present, go to next step. If Code 17 is not present, problem is intermittent. Check connectors and related circuits.
  2. Block rear wheels, and set parking brake. Raise and support front of vehicle. Connect ECU Test Harness between ECU and ECU connector. See Fig. 2 and Fig. 3 . Turn ignition on.
  3. Slowly rotate left front wheel and measure voltage between positive terminal B10 (B16 on 2.0L) and negative terminal A26 (A18 on 2.0L). See Fig. 4 and Fig. 5 . If voltmeter does not pulse, go to step 4). If voltmeter pulses from 0-5 volts, substitute a known good ECU, and recheck. If symptom/indication goes away, replace original ECU.
  4. Repair open or short in White/Blue wire between ECU and VSS. If wire is okay, replace VSS. If voltage is still not as specified, substitute a known good ECU, and recheck. If specified voltage is now present, replace original ECU.

Honda Prelude 2.0 Si 1991 - CODE 41 - O2 SENSOR HEATER 2.1L


  1. Turn ignition off. Remove appropriate fuse for 10 seconds to reset ECU. See CLEARING CODES . Start engine. Verify if CHECK ENGINE light is on and if LED is flashing Code 41. If Code 41 is present, go to next step. If Code 41 is not present, problem is intermittent. Check connections at O2 sensor, related circuits and at ECU.
  2. Turn ignition off. Disconnect 4-wire connector from O2 sensor. Measure resistance between connector terminals "A" and "B" on O2 sensor. See Fig. 7 . If resistance is 15-20 ohms, go to next step. If resistance is not 15-20 ohms, replace O2 sensor.
  3. Check for continuity from each terminal of O2 sensor to ground. If continuity exists, replace O2 sensor. If continuity does not exist, check for continuity between O2 sensor terminal "A" and terminals "C" and "D" individually. See Fig. 7 . If continuity does not exist, go to next step. If continuity exists, replace O2 sensor.

    Honda Prelude 2.0 Si 1991 - Component Locations - Heated O2 Sensor Connector Terminal ID (2.1L) https://portal-diagnostov.com/license.html
    https://portal-diagnostov.com/license.html Automotive Electricians Portal FZCO Automotive Electricians Portal FZCO
    Get access to all wiring diagrams of the carDownload PDF file

    Fig. 7: Honda Prelude 2.0 Si 1991 - Component Locations - Heated O2 Sensor Connector Terminal ID (2.1L)

  4. Measure voltage between positive Yellow/Black terminal and negative Black terminal. If battery voltage does not exist, go to step 6). If battery voltage exists, disconnect connector "A" at ECU. Measure voltage between positive Yellow/Black terminal and negative Black terminal.
  5. If there is now battery voltage, repair short in Black wire between ECU terminal A6 and O2 sensor. If battery voltage does not exist, substitute a known good ECU. If condition is corrected, replace original ECU.
  6. Measure voltage between Yellow/Black terminal and ground. If battery voltage does not exist, repair open in Yellow/Black wire between O2 sensor and main relay. If battery voltage exists, turn ignition off. Reconnect 4-wire connector to O2 sensor.
  7. Connect ECU Test Harness connector "A" to main harness, not ECU. Turn ignition on. Measure voltage between terminal A6 (+) and A23 (-). If battery voltage does not exist, repair open in Orange/Black wire between ECU terminal A6 and O2 sensor. If battery voltage exists, substitute a known good ECU. If condition is corrected, replace original ECU.

Honda Prelude 2.0 Si 1991 - CODE 43 - FUEL SUPPLY SYSTEM 2.1L


  1. Turn ignition off. Remove appropriate fuse for 10 seconds to reset ECU. See CLEARING CODES . Start engine, and warm to normal operating temperature (cooling fan comes on). Raise engine speed to 3000 RPM, and hold for 2 minutes. Verify if CHECK ENGINE light is on and if LED is flashing Code 43. If Code 43 is present, go to next step. If Code 43 is not present, problem is intermittent. Check connections at O2 sensor, related circuits and at ECU.
  2. Turn ignition off. Connect ECU Test Harness between ECU and connector. Wait at least 2 minutes with ignition off. Turn ignition on. Measure voltage between terminals D14 (+) and A26 (-) as soon as ignition is turned on. If more than .1 volt was present, go to next step. If less than .1 volt is present when ignition is first turned on, go to step 4).
  3. Disconnect 4-wire connector from O2 sensor. Measure voltage at O2 sensor connector between terminals "C" (-) and "D" (+). If voltage is more than .1 volt, replace O2 sensor. If voltage is less than .1 volt, repair open in White wire between ECU terminal D14 and O2 sensor.
  4. Disconnect 4-wire connector from O2 sensor. Measure voltage between terminals D14 (+) and A26 (-). If voltage is more than .1 volt, replace O2 sensor. If voltage is less than .1 volt, disconnect test connector "D" from main wire harness.
  5. Measure voltage between terminals D14 (+) and A26 (-). If voltage is more than .1 volt, repair short in White wire between ECU terminal D14 and O2 sensor. If voltage was less than .1 volt, substitute a known good ECU. If condition is corrected, replace original ECU.

Honda Prelude 2.0 Si 1991 - SUMMARY


If no hard fault codes (or only pass codes) are present, driveability symptoms exist or intermittent codes exist, proceed to TESTS W/O CODES article in the ENGINE PERFORMANCE Section for diagnosis by symptom (i.e., ROUGH IDLE, NO START, etc.) or intermittent diagnosis procedures.