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.