DTC code P0685 OBDII means that the Powertrain Control Module (PCM) has detected that its own power supply has failed. The code is set when there is a low voltage in the circuit that feeds it with power. Note that this code refers only to applications that use a relay to supply battery power to the PCM.
PCM power supply circuits are direct and simple. When the switch is in the on position, the relay is powered, then an electromagnet closes two contact points. Because one of these points is permanently connected to the battery power, when both points are closed, a circuit is completed that brings power to the battery, with this the PCM is fed.
The problems that accompany the diagnostic code P0685 OBD2 are:
The Powertrain Control Module (PCM) power relay may be defective.
A low voltage in the system.
Cables or connectors in bad condition.
Power switch may be in bad condition.
PCM programming error or defects.
Possible solutions of the DTC code P0685 Generic
Before the fault code P0685 OBDII you must do the following to correct it:
Consult with the Technical service bulletins (TSB) corresponding to your vehicle.
Check all cables and their respective connectors. Check if the relays may be incorrectly adjusted or if the terminals are corroded. Repair if you find problems in one of them.
Check that the battery terminals have a good connection.
On the power relay and with a multimeter, check for battery voltage at the main input terminal. Refer to the wiring diagram to locate the terminals. If no voltage is present, indicates a fuse connection is failing. Also check input terminal number two. If there is no voltage, the fuse is blown or the power switch is defective.
Inspect the ground signal. If no ground signal is present, you should check the ground locations. If all circuits are normal, you should check the output voltage that supplies voltage to the Powertrain Control Module (PCM). If there is no voltage present in these circuits, you should replace the relay as it is most likely defective.
If, on the other hand, the voltage is present, you should check the system voltage at the PCM connector. Check the cables. If there are shorts in the circuit repair them, and if necessary replace any defective components.