ECM/PCM Power Relay Control Circuit Open
The P0685 code indicates that the powertrain control module (PCM) has detected an open circuit in the control circuit for the ECM/PCM power relay. This relay supplies battery voltage to the ECM/PCM when the ignition is on. An open circuit means there is a break in the wiring or a faulty relay, preventing the ECM/PCM from receiving power, which can cause the engine to not start or run poorly.
01. Most Common Causes
- Faulty ECM/PCM power relay
- Open or shorted wiring in the relay control circuit
- Blown fuse in the relay circuit
- Corroded or loose relay socket connections
- Failed ECM/PCM (rare)
02. Symptoms You Will Notice
- Engine cranks but does not start
- Engine stalls immediately after starting
- Check Engine Light illuminated
- No communication with scan tool (if relay completely fails)
03. Step-by-Step Diagnosis
Step 1: Check for blown fuses related to the ECM/PCM relay circuit.
Step 2: Locate the ECM/PCM power relay and test it by swapping with a known good relay of the same type.
Step 3: Use a multimeter to check for battery voltage at the relay coil terminals with ignition on.
Step 4: Inspect wiring and connectors for damage, corrosion, or loose connections.
Step 5: If all else fails, test the ECM/PCM ground and power supply circuits, and consider ECM/PCM replacement.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
No, it is not safe. The vehicle may stall or fail to start, leaving you stranded. Have it diagnosed and repaired promptly.
How much does it cost to fix?▼
Typically $50-$150 for relay replacement and basic wiring repair. If the ECM/PCM needs replacement, costs can exceed $1000.