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 main power relay. This relay supplies battery voltage to the PCM and other critical components. An open circuit means the relay cannot be energized, preventing the PCM from powering up properly, which can lead to a no-start condition or intermittent electrical issues.
1. Most Common Causes
- Blown fuse in the ECM/PCM power relay circuit
- Faulty ECM/PCM power relay
- Open or shorted wiring in the relay control circuit
- Corroded or loose relay socket connections
- Failed PCM (rare)
2. Symptoms You Will Notice
- Engine cranks but does not start
- Check Engine Light illuminated
- Intermittent stalling or no power
- Other electrical components may not function
3. Step-by-Step Diagnosis
Step 1: Check for blown fuses related to the ECM/PCM power relay circuit.
Step 2: Locate the ECM/PCM power relay and test its operation using a multimeter.
Step 3: Inspect the relay socket for corrosion, damage, or loose pins.
Step 4: Check continuity in the control circuit wiring between the PCM and relay.
Step 5: If all else is fine, test the PCM output signal; replace PCM if necessary.
4. 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 immediately.
How much does it cost to fix?▼
Typically between $50 and $150 for a relay or fuse replacement. If the PCM needs replacement, costs can exceed $500.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.