P0685
ECM/PCM Power Relay Control Circuit /Open
When a vehicle powertrain module registers the fault code P0685, it points directly to an internal system malfunction identified as "ECM/PCM Power Relay Control Circuit /Open". Operating your engine under this condition may degrade long-term fuel maps.
Driver's Summary
The diagnostic trouble code P0685 indicates an active fault in the ecm/pcm power relay control circuit /open circuit or component. In practice, this fault causes engine cranks but will not start, no communication with obd2 scanner. Stop driving as soon as it is safe to do so. This fault can lead to expensive secondary damage if left unaddressed.
Symptoms
Engine cranks but will not start, no communication with OBD2 scanner
Common Causes
- Failed PCM power relay
- Blown main PCM fuse
- Open circuit in the relay control wire
- Dead battery or bad ground
How to Fix
- 1 Test and replace PCM power relay
- 2 Replace blown fuses and check for shorts
- 3 Trace and repair control wiring
- 4 Clean chassis ground connections
Technical Explanation
The ECM detects code P0685 by continuously monitoring the relevant sensor circuit against calibrated threshold values stored in its non-volatile memory. The control module samples the circuit continuously during normal operation, using both voltage level monitoring and frequency analysis to detect open circuits, shorts to ground, shorts to battery voltage, and high-resistance connections. The fault remains stored in memory even after the MIL is cleared; it becomes a confirmed DTC after failing two consecutive drive cycles, and the PCM logs a freeze frame record of the engine's exact operating state at the moment of detection.
Is It Safe to Drive?
With P0685 active, your engine or transmission is not operating within design parameters. Short-term driving may seem fine, but internal damage is accumulating — particularly to failed pcm power relay.
Mechanic's Pro Tip
Module replacement should always be the last resort for P0685 after exhaustively verifying all power supply circuits, ground connections, and communication bus wiring. Use a wiring diagram to locate all fuses, relays, and ground points for the affected module, and measure voltage drop on each ground with the circuit loaded. A module "failure" is frequently a corroded ground eyelet or a weak battery causing brownout conditions — fix these first and you'll save hundreds of dollars on an unnecessary module replacement.
Relay or fuse: $20 - $50; Wiring diagnostic: $150