When diagnosing ECU issues, specifically following a modification or chip installation, categorize the symptoms to isolate the failure point.

Common ECU Failure Symptoms#

• Blinking CEL: The Check Engine Light (CEL) displays a pattern of flashes. Refer to the diagnostic tool below to interpret these codes.
• Solid CEL: The CEL remains illuminated continuously. The service check connector fails to trigger code blinks.
• No CEL / Poor Operation: The CEL does not illuminate, but the engine exhibits poor performance or driveability issues.
• Idle Issues: Persistent idle control problems or CEL Code 14. Refer to the IACV Circuit Fix - R58 & R590 guide.

Troubleshooting Power and Ground (KOEO)#

If the vehicle fails to start, verify the ECU is receiving power and ground during the Key On, Engine Off (KOEO) state.

1. Verify ECU LED: Check if the LED on the ECU board is illuminated.
2. Sensor Voltage Test: Measure the voltage at the Throttle Position Sensor (TPS) connector. If you measure +5V, the ECU is receiving power and is properly grounded.
3. Main Relay Inspection: If no voltage is present at the sensors, inspect the Main Relay.
   • The Main Relay consists of two internal relays.
   • Upon ignition, the first relay energizes to supply power to the ECU.
   • Once the ECU is powered, it grounds the second relay to activate the fuel pump.
   • Verification: You should hear the fuel pump prime for approximately 2 seconds, and the MIL should extinguish after this interval.

Important

If the fuel pump does not prime and the MIL remains on, the ECU is likely not completing its power-up sequence. Check the Main Relay solder joints and the ECU ground connections at the thermostat housing.

Troubleshooting Solid CEL#

A solid CEL (no blinking) usually indicates that the ECU processor has failed to boot or is stuck in a reset loop.

• Check Socketing: If the ECU was recently chipped, inspect the solder joints on the 28-pin socket. Ensure there are no solder bridges between pins.
• Verify ROM: Ensure the EPROM is seated correctly and the orientation (notch) is correct.
• Clock Circuit: Verify the crystal oscillator is functioning.
• Power Supply: Check the 5V regulator on the ECU board for proper output.