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Honda Accord LX 1991 – 1991 ENGINE PERFORMANCE Self Diagnostics

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Honda Accord LX 1991 - 1991 ENGINE PERFORMANCE Self-Diagnostics


Honda Accord LX 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 this section for diagnosis by symptom (i.e., ROUGH IDLE, NO START, etc.).


Honda Accord LX 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 Accord LX 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 this section.


Honda Accord LX 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, back-up system is in operation. See TESTS W/O CODES article in this section.


Honda Accord LX 1991 - RETRIEVING CODES


With ignition off, insert jumper wire in service check connector terminal, located behind right side of dash. See Fig. 1 . Turn ignition on and count flashes on CHECK ENGINE light.

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 Accord LX 1991 - Component Locations - Locating Service Check Connector https://portal-diagnostov.com/license.html
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Fig. 1: Honda Accord LX 1991 - Component Locations - Locating Service Check Connector


Honda Accord LX 1991 - TROUBLE CODE DEFINITION



Honda Accord LX 1991 TROUBLE CODE IDENTIFICATION CHART

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


Honda Accord LX 1991 - CLEARING CODES


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


Honda Accord LX 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 Accord LX 1991 - ECU LOCATION


ECU is located under carpet in passenger-side footwell.


Honda Accord LX 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. 1. If fuse is okay, repair open circuit in Yellow wire between fuse and combination meter.
  2. Turn ignition off. Connect ECU Test Harness (07LAJ-PT3010A ) between ECU and connector. See Fig. 2 . Connect terminal A13 to ground. See Fig. 3 . Turn ignition on. If CHECK ENGINE light is now on, go to step 3). If light is off, replace light bulb or repair open in Green/Red wire between ECU terminal A13 and combination meter.

    Honda Accord LX 1991 - Component Locations - Connecting ECU Test Harness (07LAJ-PT3010A) https://portal-diagnostov.com/license.html
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    Fig. 2: Honda Accord LX 1991 - Component Locations - Connecting ECU Test Harness (07LAJ-PT3010A)

    Honda Accord LX 1991 - Component Locations - Identifying ECU Terminals https://portal-diagnostov.com/license.html
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    Fig. 3: Honda Accord LX 1991 - Component Locations - Identifying ECU Terminals

  3. 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.

Honda Accord LX 1991 - CHECK ENGINE LIGHT ON, WON'T FLASH CODE


  1. Turn ignition off. Connect jumper to service check connector terminal. See Fig. 1 . Turn ignition on. If CHECK ENGINE light illuminates and flashes code(s), proceed to appropriate self-diagnostic trouble code. If CHECK ENGINE light does not illuminate, go to next step.
  2. Remove jumper from service check connector with ignition off. Try to start engine. If engine starts, go to step 3). If engine does not start, remove and inspect ECU fuse in underhood fuse/relay block. Replace if necessary. If fuse is okay, go to step 5).
  3. Using DVOM, measure voltage between service check connector Orange/Red (+) and Green/White (-) wires. If 5 volts are present, go to step 4). If 5 volts are not present, repair short to ground in Orange/Red wire between ECU terminal C4 and service check connector.
  4. Turn ignition off. Disconnect ECU connector "A". Turn ignition on. If CHECK ENGINE light illuminates, repair short in Green/Red wire between ECU terminal A13 and CHECK ENGINE light. If CHECK ENGINE light does not illuminate, substitute a known good ECU, and recheck. If condition is corrected, replace original ECU.
  5. Check fuse No. 2 in fuse/relay block behind left side of dash. Replace if necessary. If fuse is okay, turn ignition on. Disconnect 3-wire connectors to MAP, TPS and EGR lift sensors. If CHECK ENGINE light illuminates, go to step 6). If CHECK ENGINE LIGHT does not illuminate, replace sensor causing light to illuminate.
  6. Turn ignition off. Connect ECU Test Harness (07LAJ-PT3010A ). Disconnect ECU "D" connector (not wire harness). Check continuity between terminals D19 and D20 to ground. If continuity exists, repair short in Yellow/White wire between ECU terminals D19 and D20 to TPS, MAP or EGR lift sensor. If continuity does not exist, go to next step.
  7. Reconnect all connectors. Reconnect ECU connector "D". Turn ignition on. Individually connect terminals A26 and B2 to ground. If CHECK ENGINE light still stays on after 2 seconds, go to step 8). If CHECK ENGINE light turns off after 2 seconds, repair open in Black/Red wire between ECU terminal A26 and ground or in Brown/Black wire between ECU terminal B2 and ground.
  8. Measure voltage between ECU terminals A26 (-) to B1 (+) and A25 (+). If battery voltage is present, substitute a known good ECU, and recheck. If symptoms go away, replace original ECU. If battery voltage is not present, repair open in Yellow/Black wire between ECU terminals A25 and B1 to main relay. If wire is okay, check main relay and wiring connectors at main relay.

Honda Accord LX 1991 - CODE 1 - O2 SENSOR


  1. Turn ignition off. Remove BACK-UP light fuse in underhood fuse/relay block for 10 seconds to reset ECU. Start and warm engine to normal operating temperature (cooling fan comes on).
  2. Road test vehicle with transmission in 2nd gear. Accelerate at wide open throttle for at least 5 seconds and then decelerate for at least 5 seconds with throttle completely closed.
  3. If CHECK ENGINE light illuminates and Code 1 is present, perform test for Code 43. If CHECK ENGINE light does not illuminate and Code 1 is not present, problem is intermittent. Check for poor connections and loose wires at connector No. C210 (at right shock tower), and check O2 sensor connector.

Honda Accord LX 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 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 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 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 ECU Test Harness (07LAJ-PT3010A ) between ECU and ECU harness. See Fig. 2 . Turn ignition on. Measure voltage between terminals D17 and D21. See Fig. 3 .
  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. If about 5 volts are present, repair open in Red/White wire between ECU terminal D19 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 Accord LX 1991 - CODE 4 - CRANK ANGLE SENSOR


  1. Turn ignition off. Remove BACK-UP fuse for 10 seconds to reset ECU. Start engine. Verify if CHECK ENGINE light is on and if Code 4 is present. If Code 4 is present, go to next step. If Code 4 is not present, problem is intermittent. Check connectors at distributor and related circuits.
  2. Turn ignition off. Disconnect 8-wire TDC/CRANK/CYL sensor connector nearest distributor. Measure resistance across terminals "B" and "F" at sensor. See Fig. 4 . If reading is not 260-500 ohms, replace distributor assembly. If reading is within specification, proceed to next step.
  3. Check continuity between ground and sensor terminals "B" and "F" individually. If continuity is present on either terminal, sensor is shorted to ground. Replace distributor assembly. If continuity is not present, go to next step.
  4. Reconnect 8-wire sensor connector. Connect ECU Test Harness (07LAJ-PT3010A ) to main harness side only. See Fig. 2 . Check resistance between connector terminals B15 and B16. See Fig. 3 . Resistance should be 260-500 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 B15 to ground.
  6. If continuity exists, repair short in Blue/Green wire between ECU terminal B15 and distributor connector. If continuity does not exist, substitute a known good ECU, and recheck. If condition is corrected, replace original ECU.

    Honda Accord LX 1991 - Component Locations - TDC/CRANK/CYL Sensor Connector Terminal ID https://portal-diagnostov.com/license.html
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    Fig. 4: Honda Accord LX 1991 - Component Locations - TDC/CRANK/CYL Sensor Connector Terminal ID


Honda Accord LX 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 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 ) between ECU and ECU connector. See Fig. 2 . Measure voltage between terminal D17 and terminal D21. See Fig. 3 . 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 Accord LX 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 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 and TW sensor.
  5. Disconnect 22-wire connector 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, 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 and TW sensor. If wire is okay, substitute a known good ECU, and recheck. If condition is corrected, replace original ECU.

Honda Accord LX 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 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 .
  3. Turn ignition on. Measure voltage between positive terminal D11 and negative terminal D22. See Fig. 3 . 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 and throttle sensor. On A/T models, go to step 5).
  5. Disconnect 22-wire connector 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 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 and terminal D22. If voltage is about 5 volts, repair open Yellow/White wire between ECU terminal D20 and throttle sensor. If reading is not about 5 volts, substitute a known good ECU. If condition is corrected, replace original ECU.

Honda Accord LX 1991 - CODE 8 - TDC SENSOR


  1. Turn ignition off. Remove BACK-UP fuse for 10 seconds to reset ECU. Start engine. Verify if CHECK ENGINE light is flashing Code 8. If Code 8 is present, go to next step. If Code 8 is not present, problem is intermittent. Check connectors at distributor and related circuits.
  2. If CHECK ENGINE light is on, turn ignition off. Disconnect 8-wire sensor connector at distributor. Using an ohmmeter, measure resistance between terminals "C" and "G". See Fig. 4 . If reading is not 260-500 ohms, replace distributor assembly. If reading is within specification, proceed to next step.
  3. Check continuity between terminals "C" and "G" individually. If continuity exists at either terminal, sensor is shorted to ground. Replace distributor. If continuity does not exist, proceed to next step.
  4. Reconnect sensor connector. Attach ECU Test Harness to main wire harness side only. See Fig. 2 . Using an ohmmeter, check resistance between connector B13 and B14 terminals. See Fig. 3 . Resistance should be 260-500 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 B13 to ground.
  6. If continuity exists, repair short in Orange/Blue wire between ECU terminal B13 and sensor connector. If continuity does not exist, substitute a known good ECU, and retest. If condition is corrected, replace original ECU.

Honda Accord LX 1991 - CODE 9 - CYL SENSOR


  1. Turn ignition off. Remove BACK-UP fuse for 10 seconds to reset ECU. Start engine. Verify if CHECK ENGINE light is flashing Code 9. If Code 9 is present, go to next step. If Code 9 is not present, problem is intermittent. Check connectors at distributor and related circuits.
  2. Turn ignition off. Disconnect 8-wire sensor connector near distributor. Measure resistance between terminals "D" and "H" at sensor harness connector. See Fig. 4 . If reading is not 260-500 ohms, replace distributor assembly. If reading is within specification, proceed to next step.
  3. Check continuity between ground and terminals "D" and "H" individually. If continuity exists at either terminal, replace distributor 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. See Fig. 2 . Check resistance between connector terminals B11 and B12. See Fig. 3 . Resistance should be 260-500 ohms. If resistance is not as specified, repair open in Orange and/or White wires.
  5. If resistance is within specification, check continuity between terminal B11 and ground. If continuity is present, repair short in Orange wire between ECU terminal B11 and distributor connector.
  6. If continuity is not present, substitute a known good ECU, and recheck. If condition is corrected, replace original ECU.

Honda Accord LX 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 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 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 and TA sensor. If wire is okay, substitute a known good ECU, and recheck. If condition is corrected, replace original ECU.

Honda Accord LX 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 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 Pink wire between EGR solenoid control valve and ECU terminal A11. 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. 2. 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 Pink 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 (07LAJ-PT3020A ) 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 and EGR valve or Green/White wire between ECU terminal D22 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 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 . Start engine, and allow to idle. Measure voltage between ECU positive terminal D12 and negative terminal D22. See Fig. 3 . If approximately 1.2 volts are not present, repair open in White/Black or Yellow wire between ECU terminal D12 and EGR valve lift sensor.
  14. If 1.2 volts are present, connect jumper between terminals A11 and A26. If engine does not stall, repair open in Pink wire between ECU terminal A11 and EGR control solenoid valve. If engine stalls, substitute a known good ECU. If condition goes away, replace original ECU.

Honda Accord LX 1991 - CODE 13 - ATMOSPHERIC PRESSURE (PA) SENSOR


  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 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 Accord LX 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 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 Black/Blue 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 Black/Blue wire between ECU terminal A9 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 (+) and A23 (-). If voltage does not exist, repair open in Black/Blue wire between ECU terminal A9 and EACV.
  8. If battery voltage exists, connect and disconnect terminal A9 and A23. 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 Accord LX 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 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 2-wire connector at distributor. 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 . Turn ignition on. Individually measure voltage between negative terminal A26 and positive terminals A21 and A22. See Fig. 3 .

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

  4. 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 Yellow/Green wire between distributor and ECU terminals A21 and A22. If wires are okay, replace ignitor unit.

Honda Accord LX 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 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.
  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 . Turn ignition on. Measure voltage between negative terminal A23 and following positive terminals. See Fig. 3 .
    • 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 Accord LX 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 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 . Turn ignition on.
  3. Slowly rotate left front wheel and measure voltage between positive terminal B10 and negative terminal A26. See Fig. 3 . 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 Orange 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 Accord LX 1991 - CODE 20 - ELECTRICAL LOAD DETECTOR


  1. Turn ignition off. Remove appropriate fuse for 10 seconds to reset ECU. See CLEARING CODES . Start engine, and turn on headlights with engine at idle. Verify if CHECK ENGINE light is flashing Code 20. If Code 20 is present, go to next step. If Code 20 is not present, problem is intermittent. Check for poor connections and loose wires.
  2. Turn ignition off. Open lid of main fuse panel in engine compartment. Disconnect 3-wire electrical load detector. Turn ignition on. Measure voltage between Black/Yellow (+) and Black (-) wire terminals of harness. If battery voltage is present, go to step 4).
  3. If battery voltage is not present, measure voltage between Black/Yellow wire and ground. If battery voltage is now present, repair open in Black wire between connector and ground. If battery voltage is not present, repair open Black/Yellow wire between fuse No. 2 and connector.
  4. Turn ignition off. Connect ECU Test Harness between ECU and ECU connector. See Fig. 2 . Disconnect connector "D" from ECU side only. Connect Green/Red terminal of load detector harness connector to ground. Check for continuity between ECU terminal D10 and ground. See Fig. 3 . If continuity does not exist, repair open Green/Red wire between ECU connector terminal D10 and load detector connector.
  5. If continuity exists, reconnect connector "D" to ECU. Turn ignition on. Measure voltage between Green/Red (+) and Black (-) terminals of load detector harness. If 4.5-5 volts is present, go to step 7). If 4.5-5 volts is not present, disconnect connector "D" from main wiring harness, not ECU. Measure voltage between positive terminal D10 and negative terminal A26.
  6. If voltage is now 4.5-5 volts, repair short in Green/Red wire between ECU terminal D10 and load detector connector. If voltage is not 4.5-5 volts, substitute a known good ECU. If correct voltage is now available, replace original ECU.
  7. Reconnect load detector connector. Start engine. Measure voltage between positive terminal D10 and negative terminal A26 under following conditions:
    • Headlight switch in 1st pos. - should be 2.5-3.5 volts.
    • Headlight switch in 2nd pos. - should be 1.5-2.5 volts.
  8. If voltage is not as specified, replace faulty electrical load detector. If voltage is as specified, substitute a known good ECU, and retest. If condition is corrected, replace original ECU.

Honda Accord LX 1991 - CODE 30 - A/T FI SIGNAL A


  1. Turn ignition off. Remove BACK-UP fuse for 10 seconds to reset ECU. Road test vehicle. Drive for several miles so transmission upshifts and downshifts several times. Verify if CHECK ENGINE light is on or flashing Code 30. If Code 30 is present, go to next step. If Code 30 is not present, problem is intermittent and system is okay at this time.
  2. Turn ignition off. Connect ECU Test Harness between ECU and ECU connector. See Fig. 2 . Disconnect 22-wire connector from A/T control unit. Check for continuity between ECU terminal B3 and ground. See Fig. 3 .
  3. If continuity exists, repair short in White/Red wire between ECU terminal B3 and A/T control unit. If continuity does not exist, check for continuity on White/Red wire between ECU terminal B3 and A/T control unit connector.
  4. If continuity does not exist, repair open in White/Red wire between ECU terminal B3 and A/T control unit. If continuity exists, substitute a known good ECU. If condition is corrected, replace original ECU.

Honda Accord LX 1991 - CODE 31 - A/T FI SIGNAL B


  1. Turn ignition off. Remove BACK-UP fuse for 10 seconds to reset ECU. Road test vehicle. Drive for several miles so transmission upshifts and downshifts several times. Verify if CHECK ENGINE light is on or flashing Code 31. If Code 31 is present, go to next step. If Code 31 is not present, problem is intermittent and system is okay at this time.
  2. Turn ignition off. Connect ECU Test Harness between ECU and ECU connector. See Fig. 2 . Disconnect 22-wire connector from A/T control unit. Check for continuity between ECU terminal B4 and ground. See Fig. 3 .
  3. If continuity exists, repair short in Green wire between ECU terminal B4 and A/T control unit. If continuity does not exist, check for continuity on White/Red wire between ECU terminal B4 and A/T control unit connector.
  4. If continuity does not exist, repair open in Green wire between ECU terminal B4 and A/T control unit. If continuity exists, substitute a known good ECU. If condition is corrected, replace original ECU.

Honda Accord LX 1991 - CODE 41 - O2 SENSOR HEATER


  1. Turn ignition off. Remove appropriate fuse for 10 seconds to reset ECU. See CLEARING CODES . Start engine. Verify if CHECK ENGINE light 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. 5 . 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. 5 . If continuity does not exist, go to next step. If continuity exists, replace O2 sensor.

    Honda Accord LX 1991 - Component Locations - Heated O2 Sensor Connector Terminal ID https://portal-diagnostov.com/license.html
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    Fig. 5: Honda Accord LX 1991 - Component Locations - Heated O2 Sensor Connector Terminal ID

  4. Measure voltage between positive Yellow/Black terminal and negative Orange/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 Orange/Black terminal.
  5. If there is now battery voltage, repair short in Orange/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 Accord LX 1991 - CODE 43 - FUEL SUPPLY 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 (cooling fan comes on). Raise engine speed to 3000 RPM, and hold for 2 minutes. Verify if CHECK ENGINE light 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 Accord LX 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 this section for diagnosis by symptom (i.e., ROUGH IDLE, NO START, etc.) or intermittent diagnosis procedures.