P0685 is a generic OBD-II diagnostic trouble code indicating the Powertrain Control Module detected an open or malfunctioning control circuit for the ECM/PCM power relay. This relay supplies main operating voltage to the vehicle’s primary computer.
This code identifies an electrical circuit condition rather than confirming a single failed component. The underlying cause could involve a blown fuse, wiring damage, a failed relay, or PCM issues, requiring targeted electrical diagnosis.
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Key Takeaways
- P0685 indicates the control module detected an electrical fault, such as an open circuit, in the ECM/PCM power relay control circuit.
- Possible causes include a failed power relay, blown main fuse, damaged wiring, corroded connectors, or internal PCM faults.
- Diagnosis requires verifying battery voltage, testing the power relay, and checking circuit continuity between the relay and the control module.
- Repairs depend on the confirmed root cause and may involve relay replacement, wiring repair, or fuse replacement.
P0685 Code Description and Implications
The P0685 code indicates that the Powertrain Control Module (PCM) has detected an electrical fault in the control circuit for the main ECM/PCM power relay.
This relay is responsible for supplying battery voltage to the PCM when the ignition is turned on, allowing the computer to boot up and manage engine functions.
An open circuit condition means the PCM cannot detect the expected voltage feedback from the relay control circuit, or the circuit is broken.
Because P0685 is a generic code, this fundamental electrical monitoring logic applies across various vehicles from multiple manufacturers.
However, the exact relay location, wiring diagrams, and diagnostic troubleshooting steps vary considerably depending on the specific vehicle application.
When diagnosing OBD2 Codes related to main power relays, generic diagnostic paths cannot account for manufacturer-specific fuse box layouts or wiring routings.
Common Causes of the P0685 Code
The underlying causes for an ECM/PCM power relay control circuit code primarily involve the relay itself, the power supply, or the control wiring.
Failed Power Relay: The internal contacts or the electromagnetic coil of the main power relay may fail, preventing it from closing the circuit and sending power to the PCM.
Blown Fuses or Fusible Links: A blown main fuse or fusible link supplying power to the relay will prevent the circuit from operating, triggering an open circuit code.
Wiring and Connector Issues: Damaged wiring, chafed insulation, or corroded connectors in the control circuit between the PCM and the relay can interrupt the electrical signal.
Faulty PCM/ECM: In uncommon instances, an internal driver failure within the PCM may prevent it from properly grounding or controlling the relay circuit.
Symptoms Associated With P0685
The prominent symptom of a P0685 code is a no-start condition, as the PCM will not receive the power necessary to manage fuel and spark.
If the relay is intermittently failing, the engine may stall unexpectedly while driving or fail to crank when the key is turned.
The Check Engine Light will illuminate, and the vehicle may enter a fail-safe or limp mode if the engine manages to run.
Some applications may also experience a complete loss of communication with the OBD-II port, as the PCM is completely unpowered.
Diagnosing the P0685 Trouble Code
Diagnosis must begin by confirming the exact factory service information and wiring diagrams for the vehicle’s make and model.
Start by checking the main battery voltage and inspecting the primary fuses and fusible links in the under-hood fuse box.
Next, locate the ECM/PCM power relay and test it according to the manufacturer’s procedure.
This may involve swapping it with an identical, known-good relay to see if the condition changes.
Use a digital multimeter to verify that the relay is receiving battery voltage on the power supply pin.
Verify that the PCM is providing the necessary ground or power signal on the control pin.
If the external wiring and power supply test correctly, the fault may lie within the PCM itself, requiring module diagnosis.
Repairing the P0685 Issue
Repairs must follow confirmed diagnosis.
Never replace the PCM solely based on the presence of the code without verifying the relay and circuit integrity.
If a blown main fuse is found, replace the fuse and investigate the cause of the overload before clearing the codes.
If the ECM/PCM power relay fails electrical testing, replace the relay with a component meeting the correct vehicle specification.
If wiring damage or connector corrosion is found, repair the harness and secure the connections before re-evaluating the system.
If all external circuits and the relay test correctly, follow the manufacturer’s diagnostic procedure to evaluate the PCM for internal failure.
Cost Considerations for P0685 Repairs
Repair costs vary considerably depending on the confirmed root cause and the specific vehicle application.
Replacing a blown fuse or a standard power relay involves basic parts costs and minimal labor.
Repairing damaged wiring in the engine bay harness requires standard electrical labor and materials.
If the diagnosis confirms the PCM requires replacement and programming, the parts and labor costs will be higher.
Cost depends on diagnostic time, component accessibility, required parts, programming requirements, and labor.
Prevention Strategies for Future Occurrences
Preventing main power relay circuit codes requires attention to the vehicle’s primary electrical system and under-hood environment.
Ensuring the battery terminals are clean and securely tightened prevents voltage drops that can stress relay coils and control modules.
Inspecting the under-hood fuse box for moisture intrusion or corrosion can help identify developing connection issues before they cause an open circuit.
Keeping the engine bay clean and protecting exposed wiring harnesses from excessive heat and abrasion helps maintain circuit integrity.
Troubleshooting Tips and Best Practices
Always check for Technical Service Bulletins related to P0685 for your specific vehicle application.
Manufacturers may release updated relay part numbers, revised fuse box assemblies, or software updates to address known power supply faults.
Do not assume the PCM has failed simply because it controls the relay circuit.
A simple blown fuse, a corroded relay socket, or a failed relay are more probable causes of this specific open circuit code.
If the vehicle recently underwent engine or electrical work, verify that all main ground straps and power cables were reconnected securely.
More OBD-II Codes
| P0675 | P0676 | P0677 | P0678 |
| P0683 | P0688 | P0690 | P0691 |
| P0700 | P0703 | P0704 | P0705 |
| P0600 | P0601 | P0602 | P0603 |
| P0604 | P0605 | P0560 | P0562 |
| P0563 | P0606 | P0607 | P0641 |
Frequently Asked Questions
Is the P0685 code generic or manufacturer-specific?
P0685 is a generic SAE-defined OBD-II code. Its fundamental definition regarding the ECM/PCM power relay control circuit applies across various manufacturers, though exact diagnostic steps and relay locations vary by vehicle application.
Can a bad battery cause a P0685 code?
A completely dead battery prevents the PCM from powering up, but P0685 specifically indicates an open or faulty control circuit within the relay system itself, rather than just a lack of overall battery voltage.
Will disconnecting the battery clear the P0685 code?
Disconnecting the battery may temporarily clear the code, but it does not repair the underlying electrical fault. It also erases valuable freeze frame data and may reset readiness monitors, complicating professional diagnosis.
Can I drive my vehicle with a P0685 code?
Driving is impossible or unsafe with this code. A faulty main power relay typically causes a no-start condition or causes the engine to stall unexpectedly while driving due to a sudden loss of PCM power.
Is the ECM/PCM power relay easy to replace?
Accessibility varies by application. On numerous vehicles, the main power relay is located in the under-hood fuse box and is easily accessible for a simple plug-and-play replacement without requiring special tools.
What Should You Do About the P0685 Code?
Confirm the exact factory definition and diagnostic procedure for your specific vehicle application before attempting repairs.
Do not assume the PCM has failed based solely on the presence of the diagnostic trouble code.
Inspect the main fuses, test the power relay, and verify circuit continuity to rule out simple electrical faults.
Follow the applicable manufacturer diagnostic workflow to isolate the relevant system and components before replacing any control modules.
