P0685 code definition:
The PCM (power train control module) receives data about the vehicle's operating conditions from various areas in the system. In order to work properly, the PCM is supplied with power by a relay, which uses a battery voltage signal, a ground signal, and an ignition switch input signal. The voltage in the relay coil control circuit is normally low. If you see trouble code P0685, it means that the PCM's fault detection circuit has detected voltage from the relay that measures above the normal parameters
What causes the P0685 code?
A defective PCM power relay
A blown fuse or fusible link
Corroded or damaged wiring or wiring connectors
Faulty ignition switch
Partially or totally disconnected electrical terminal at the ignition switch
What are the symptoms of the P0685 code?
Symptoms may range from just the service engine soon lamp and a stored code to a no start/no crank condition. In some instances, the service engine soon lamp will be totally disabled when the PCM relay is defective.
Common mistakes when diagnosing the P0685 code:
A rushed diagnosis will often lead to unnecessary PCM replacement. Technicians report that shortcomings in the voltage supply to the PCM relay are more common in late model vehicles than in earlier vehicles equipped with the OBD-II systems.
What repairs can fix the P0685 code?
Repair of shorts or bad terminals or connections
Replacement of the power train control module relay
Under hood junction block replacement (fuse box)
Replacement of battery cables or connectors