ECM/PCM Power Relay Control Circuit/Open
The ECM/PCM power relay isn't responding to control signals from the engine computer, like a light switch that won't turn on the power supply. This prevents the computer from managing engine startup and operation.
The ECM monitors voltage and continuity in the power relay control circuit, which should show proper signal voltage when the ignition is turned on. The ECM expects to detect a valid ground path and control signal to activate the relay that supplies power to critical engine systems. If the circuit is open or the relay doesn't respond to the control signal, a fault is triggered.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Relay Control Signal Voltage | 12V or valid PWM signal present | 0V or open circuit detected |
| Relay Coil Ground Path | Continuous path with <1Ω resistance | Open circuit or >5Ω resistance |
Code P0685 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, P0685 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.