P0685 is set when the Powertrain Control Module (PCM) detects an open in the control circuit of the ECM/PCM power relay. The code means the relay that supplies voltage to the PCM is not being commanded correctly or the wiring to the relay is interrupted. As a result, the PCM may not receive power, causing starting or drivability issues.
The relay itself can fail mechanically or electrically, causing an open control circuit. This is the most common source of the code.
A missing or burned‑out fuse prevents the relay coil from being powered, which the PCM sees as an open circuit.
Breaks, shorts, or corrosion in the harness between the PCM and relay interrupt the control signal.
A weak ground or low supply voltage can keep the relay from engaging, mimicking an open circuit.
If the relay is commanded by the ignition switch, a defective switch can prevent the command from reaching the relay.
In very few cases the PCM itself does not generate the control signal correctly. This should be investigated only after the above items are verified.
In most vehicles the PCM is not the culprit. The code is usually caused by a bad relay, a blown fuse, or wiring problems. Replacing the PCM should be a last resort after those items have been ruled out.
A weak battery can cause low system voltage, which may prevent the relay from engaging, but the code specifically points to an open control circuit. Charging or replacing the battery may resolve a low‑voltage symptom, but the underlying open circuit must still be addressed.
The code indicates that the engine may not stay running or could stall, especially if the PCM loses power. It is advisable to diagnose and repair the issue promptly to avoid being stranded.
Because the PCM is the “brain” that monitors the circuit, some technicians incorrectly assume it is faulty. In reality, the relay, fuse, and wiring are far more likely causes, and the PCM should only be replaced after those have been verified as good.