Stored codes are indicated by the ECU's flashing LED. If the "CHECK ENGINE" warning light was or remains illuminated, remove the floor mat on the front right passenger side. The ECU is located under the protective cover; the LED can be seen through a small window without removing the ECU cover. Set the ignition key to the "ON" position; the LED will flash to indicate all stored codes. Additionally, if the service connector is shorted on the Accord or 2.1L Prelude, the "CHECK ENGINE" light will display stored codes in the same manner. The 2-pin service connector is located under the right side of the instrument panel on the Accord or on the firewall on the Prelude, next to the transmission.
Codes 1 through 9 are indicated by short-term flickering; in two-digit codes, the first most significant digit is displayed in a series of longer flashes, followed by shorter flashes of the least significant digit. For example, code 43 would be indicated for "A" by long flashes, and "3" by short flashes. The codes are separated by pauses between transmissions. The position of the codes during the output can be useful in diagnostic work. Multi-digit codes appearing in isolation indicate unique cases; if the LED displayed the code 1-1-1-pause-9-9-9, this indicates two problems or problems observed at different times. Alternating display, such as 1-9-1-9-1, indicates problems occurring simultaneously.
When counting the flashes to determine the codes, a code may be encountered that is not valid for the vehicle. In this case, re-count to confirm the correct count. If necessary, turn the ignition key to the "OFF" position, return to the initial state and start counting again. If the code is still not standard for the vehicle, the ECU must be replaced.
The S or S ₄ light may illuminate along with the "CHECK ENGINE" light on Accorde and Prelude with automatic transmission if codes 6, 7 or 17 are stored in memory. In this case, follow the troubleshooting procedures primarily for the fuel injection system. Clear the memory after repair, then recheck the S or S ₄ light.
Clearing codes in memory
Stored codes are cleared from memory when power is removed from the ECU. Powering down also resets (or clears) the memory used for other electronics such as the clock and radio. For this reason, always make notes of the radio settings before clearing the system.
Since disconnecting the battery will also clear the memory, this is not a recommended procedure.
The memory can be cleared by removing the appropriate fuse for at least 10 seconds. In all cases, the memory-clearing fuse is located under the hood in the fuse and relay box. On the Accord, this is the BACK-UP fuse; on the Prelude it is the CLOCK fuse.
Self-diagnosis indicator flashing
Trouble Codes - 1990-91 Accord
| 0 | ECU |
| 1 | Oxygen content |
| 3 | Manifold Absolute Pressure |
| 4 | Crankshaft rotation angle |
| 6 | Coolant temperature |
| 7 | Throttle angle |
| 8 | TDC position |
| 9 | Cylinder Position Number 1 |
| 10 | Intake air temperatures |
| 12 | Exhaust gas recirculation system |
| 13 | Atmospheric pressure |
| 14 | Electronic air control |
| 15 | Ignition output signal |
| 14 | Fuel injector |
| 17 | Vehicle speed sensor |
| 20 | Electrical Load Detector |
| 30 | A/T FI signal A |
| 31 | A/T FI signal B |
| 41 | Oxygen heater sensor |
| 43 | Fuel supply system |
Trouble Codes - 1990-91 Prelude
| 0 | ECU |
| 1 | Oxygen content A*, Oxygen content** |
| 2* | Oxygen content B |
| 3 | Manifold Absolute Pressure |
| 5 | |
| 4 | Crankshaft rotation angle |
| 6 | Coolant temperature |
| 7 | Throttle angle |
| 8 | TDC position |
| 9 | Cylinder position No.1 |
| 10 | Intake air temperatures |
| 12 | Exhaust gas recirculation system |
| 13 | A1atmospheric pressure |
| 14 | Electronic air control |
| 15 | Ignition output signal |
| 16 | Fuel injector |
| 17 | Vehicle speed sensor |
| 41** | Oxygen heater sensor |
| 43** | Fuel supply system |
One asterisk indicates a 2.0L engine.; the double asterisk indicates a 2.1L engine.