When a sensor or system on a Honda engine malfunctions, the Engine Control Unit (ECU) illuminates the dashboard Check Engine Light (CEL/MIL) and stores a diagnostic trouble code (DTC) in its memory.

How to Retrieve Diagnostic Codes#

The method for retrieving codes depends on the vehicle's OBD generation.

OBD0 Systems (1988–1991)#

OBD0 ECUs display codes via an LED mounted directly on the ECU circuit board:

1. Turn the ignition key to the ON position.
2. Locate the ECU (typically under the passenger footwell carpet or seat).
3. Observe the red LED through the circular viewing window in the ECU's metal case.
4. Counting: Count the flashes of the red LED. A code 9 is represented by 9 quick flashes.

OBD1 Systems (1992–1995)#

OBD1 systems flash the dashboard Check Engine Light (CEL) when the Service Connector is jumped.

1. Locate the SCS Connector: On the passenger side, under the glove box, find a green cover housing a blue 2-pin connector.
2. Jump the Connector: With the ignition OFF, connect the two pins using a paperclip or jumper wire.
3. Read the CEL: Turn the key to the IGN position (do not start the engine). The CEL will begin to flash.
4. Interpret the Flashes:
   • Long flashes (1.0 sec): Tens digit (e.g., 2 long = 20).
   • Short flashes (0.5 sec): Units digit (e.g., 1 short = 1).
   • Example: LONG-LONG-SHORT-PAUSE is Code 21 (VTEC Solenoid).

Tip

If the CEL remains solid, the ECU is not entering diagnostic mode. Verify the jumper wire is making solid contact with both pins.

OBD2 Systems (1996–2001)#

OBD2 systems utilize a standardized 16-pin Data Link Connector (DLC) located under the driver's side dashboard.

• Retrieval: Requires a standard OBD2 scan tool or code reader.
• Codes: Uses alphanumeric codes (e.g., P0420).
• Legacy Support: Many OBD2 Hondas still support the "flash" method via the 2-pin SCS connector for basic engine codes.

Master Diagnostic Code Reference#

Use the tool below to search for engine (ECU) and transmission (TCU) codes.

Troubleshooting Procedures#

1. Clear the Memory: Remove the BACK UP fuse (7.5A) in the engine bay fuse box for 30 seconds to reset the ECU.
2. Confirm the Fault: Drive the vehicle. If the CEL returns, the fault is "hard" and requires repair.
3. Test Wiring: Most "sensor" codes are caused by frayed wiring or loose connectors, especially in older Honda harnesses. Always perform a continuity test before replacing a sensor.
4. Verify VSS: If you have Code 17 (VSS), VTEC will not engage and the speedometer may be inoperative.

Warning

Do not attempt to clear codes by disconnecting the battery for extended periods, as this may cause the ECU to lose idle learning parameters, leading to poor idle quality.