P0685
UniversalThe Powertrain Control Module (PCM) or Engine Control Module (ECM) power relay control circuit is open, indicating a fault where the relay cannot properly supply power to the module.
High SeverityThis code signifies that the Engine Control Module (ECM) or Powertrain Control Module (PCM) is not receiving consistent power due to an open circuit in its power relay control. When this relay fails or its circuit is open, the vehicle's computer system may not power on, leading to a no-start condition or the engine stalling while driving.
⚠️ Before You Begin (Safety First)
Always disconnect the battery's negative terminal before inspecting electrical components to prevent accidental short circuits or electrical shock. Ensure the vehicle is safely parked with the parking brake engaged when performing any diagnostic or repair work.
Common Symptoms
- Check Engine Light illuminated
- Engine cranks but does not start
- Engine stalls unexpectedly while driving
- No power to the ECM/PCM (confirmed by diagnostic tools)
- Other modules may not communicate with the scan tool
Common Causes
- Faulty ECM/PCM power relay
- Open or short circuit in the ECM/PCM power relay control circuit
- Blown fuse supplying power to the ECM/PCM relay
- Corroded or loose electrical connectors to the relay or PCM/ECM
- Faulty ECM/PCM (less common, but possible if all other checks pass)
Frequently reported on: General/Universal
Diagnostic Steps
- Check all fuses related to the ECM/PCM and its power relay, typically located in the under-hood fuse box. Replace any blown fuses.
- Locate the ECM/PCM power relay, usually found in the under-hood fuse box or another main fuse block.
- Perform a visual inspection of the relay and its harness for any signs of corrosion, damage, or loose connections.
- Test the ECM/PCM power relay. This can be done using a multimeter to check for continuity and resistance, or by temporarily swapping it with a known good relay of the same type (if available and safe for a brief test).
- Use a multimeter to check for proper voltage and ground signals at the relay's control circuit terminals when the ignition is on.
- Inspect the wiring leading to and from the ECM/PCM power relay for any breaks, chafing, or signs of rodent damage along its path.
👨🔧 When to Call a Professional
If you have checked all fuses and replaced the PCM/ECM power relay but the problem persists, or if you are uncomfortable testing electrical circuits with a multimeter, it is best to consult a professional mechanic. This code can indicate complex wiring issues or, in rare cases, an internal PCM/ECM fault.
Commonly Asked Questions
What is the ECM/PCM power relay?
The ECM/PCM power relay is an electrical switch that provides power to the vehicle's main computer, the Engine Control Module (ECM) or Powertrain Control Module (PCM), allowing it to operate correctly.
Can I drive with a P0685 code?
No, it is generally not recommended to drive with a P0685 code. If the ECM/PCM is not receiving power, the engine may not start or could stall unexpectedly at any time, creating a dangerous situation.
How can I test the ECM/PCM power relay?
You can test the relay by checking its continuity and resistance with a multimeter, or by applying battery voltage to its control terminals to see if the internal switch 'clicks' and creates continuity across its load terminals. Swapping it with a known good relay is also a quick diagnostic step.
Could a P0685 code mean my ECM/PCM is bad?
While less common, a faulty ECM/PCM can sometimes cause a P0685 if its internal power circuit fails. However, it is crucial to thoroughly rule out issues with the relay, fuses, and wiring first, as these are much more frequent causes of this code.
Where is the ECM/PCM power relay usually located?
The ECM/PCM power relay is typically located in the main fuse box under the hood of your vehicle, or sometimes in a smaller fuse/relay box within the engine compartment or occasionally inside the passenger cabin. Consult your vehicle's owner's manual for its exact location.