Malfunction Indicator Lamp (MIL) Control Circuit Malfunction
The check engine light circuit isn't working properly—the ECU can't turn on the warning lamp or control it correctly. Think of it like a dashboard warning light that's supposed to illuminate when there's a problem, but the electrical connection between the computer and the light is broken or faulty.
The ECM sends a control signal to the MIL circuit to ground the lamp when diagnostic faults are detected. The ECU monitors the voltage and current response on this circuit to confirm the light is functioning. If resistance, open circuits, or short-to-voltage conditions exist, the ECU recognizes the control circuit has failed.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| MIL control signal voltage | 0V when lamp ON (grounded), 12V when lamp OFF | Stuck high or low voltage, no response to ECU command |
| MIL circuit resistance | Less than 10 ohms when grounded | Open circuit (infinite resistance) or short-to-power |
Code P0654 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P0654 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.