ECM/PCM Power Relay Control Circuit Open
P0685 indicates an open circuit in the PCM/ECM power relay control circuit, meaning the PCM is not receiving proper battery voltage. This typically causes a no-start condition and prevents the engine from running at all. If the engine does start with this code present, it suggests a serious PCM malfunction that requires immediate diagnosis.
What You Might Notice
- Engine will not start
- No dashboard lights or gauges illuminated
- No fuel pump priming sound
- No ignition coil or injector operation
- Check engine light or no electrical response from vehicle controls
Most Common Causes
- 1
Defective PCM Power Relay
The relay that supplies battery voltage to the PCM has failed internally or is stuck open. This is the most common cause and directly prevents power from reaching the PCM.
- 2
Blown Fuse or Fusible Link
A fuse or fusible link in the PCM power circuit has blown, cutting off voltage supply to the module. This can occur due to an electrical surge or short circuit.
- 3
Corroded or Damaged Wiring and Connectors
Corrosion, broken wires, or loose connectors in the power circuit between the battery, relay, and PCM prevent proper electrical connection. This is especially common near the relay itself.
- 4
PCM Failure or Programming Error
If the engine starts and runs despite this code, the PCM itself may be defective or have corrupted programming that causes false circuit detection.
How to Diagnose It
- 1
Check Defective PCM Power Relay — The relay that supplies battery voltage to the PCM has failed internally or is stuck open. This is the most common cause and directly prevents power from reaching the PCM.
- 2
Check Blown Fuse or Fusible Link — A fuse or fusible link in the PCM power circuit has blown, cutting off voltage supply to the module. This can occur due to an electrical surge or short circuit.
- 3
Check Corroded or Damaged Wiring and Connectors — Corrosion, broken wires, or loose connectors in the power circuit between the battery, relay, and PCM prevent proper electrical connection. This is especially common near the relay itself.
- 4
Check PCM Failure or Programming Error — If the engine starts and runs despite this code, the PCM itself may be defective or have corrupted programming that causes false circuit detection.
How to Fix It
- 1
Replace PCM Power Relay
Locate the PCM power relay (typically in the engine bay fuse/relay box) and replace it with a new relay of the same specification. Test that the engine now starts and the code clears.
- 2
Check and Replace Blown Fuses
Inspect the fuse box for blown fuses in the PCM power circuit. Replace any blown fuses with the correct amperage rating and inspect the circuit for shorts or overload conditions.
- 3
Repair Wiring and Connectors
Inspect all wiring and connectors between the battery, relay, and PCM for corrosion, damage, or loose connections. Clean corroded contacts with electronic cleaner, repair damaged wires, and ensure all connectors are fully seated and secure.
- 4
Test and Replace PCM if Necessary
If the above repairs do not resolve the code and the engine refuses to start, have the PCM tested by a professional diagnostic center. If testing confirms PCM failure, replace with a new or remanufactured unit and reprogram as needed.
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