The P0685 code on a 2011 Honda Pilot indicates a fault in the ECM/PCM Power Relay Control Circuit, typically pointing to a problem with the power supply to the engine control module. When the PCM doesn’t receive proper power, the vehicle can have starting issues, poor performance, or a no-start condition.
In this article, we break down what the code means for the Honda Pilot, common underlying causes, how to diagnose the issue, and practical steps for fixing it. The goal is to help owners understand the fault and decide whether professional service is needed.
Understanding P0685 in the 2011 Honda Pilot
P0685 is a diagnostic trouble code that signals a fault in the circuit that powers the ECM/PCM via its relay. In practical terms, the engine control module may not receive consistent power, which can prevent proper ignition timing, fuel delivery, and other critical engine-management functions. The result can range from a no-start condition to intermittent stalling or misbehavior, and the check engine light will typically be on or flashing.
Common causes
The following are among the most frequent culprits behind P0685 on a 2011 Honda Pilot. If you’re troubleshooting, these are the targets to inspect first.
- Faulty ECM power relay or its coil
- Blown fuse protecting the ECM power circuit
- Poor or corroded ECM ground and/or other grounding points
- Failing ECM/PCM unit
- Aftermarket wiring or modifications that affect the ECM power circuit
These issues can prevent stable power from reaching the PCM or create intermittent drops that trigger the diagnostic code. In many cases, the problem is mechanical (relay or fuse) or related to wiring, rather than a full ECM failure, but both are possibilities that require verification.
Diagnosis and fixes
To accurately diagnose P0685, a systematic check of power, relays, fuses, and connectors is needed. The steps below outline a practical approach that many technicians follow. Always exercise caution when working near the battery and electrical system.
- Verify the code and any related DTCs with a scan tool. Note if P0685 appears alone or with other electrical or communication codes, as this can guide the investigation.
- Check the vehicle’s battery and charging system. Ensure the battery is healthy and that voltage is stable (typically around 12.6V with the engine off and 13.8–14.8V when running). Low voltage can cause PCM power issues and set codes.
- Inspect fuses and the ECM power relay circuit. Locate the fuse that feeds the ECM power circuit and the ECM power relay in the under-hood fuse/relay box. Look for a blown fuse or a stuck/open relay, and replace as needed.
- Test the ECM power relay and its coil. If you have a similar relay in the fuse box, you can swap them temporarily to see if the code behavior changes. Alternatively, bench-test the relay coil with a multimeter to confirm it energizes and opens/closes correctly.
- Inspect ECM electrical connections and grounds. Disconnect and inspect the ECM connectors for bent pins, corrosion, and dirt. Ensure the grounds (including the engine/block ground and any PCM grounds) are clean, tight, and free of corrosion.
- Check the wiring harness between the battery, fuse box, relay, and ECM for damage. Look for damaged insulation, frayed wires, or chafing that could cause intermittent power loss or short to ground.
- If power delivery to the ECM is verified and all circuits test within spec, consider the possibility of an internal ECM fault. In such cases, replacement or reprogramming may be required, typically by a dealer or qualified automotive electrical specialist.
After completing these diagnostic steps, you should have a clearer idea of whether the fault lies with a relay, fuse, wiring, grounding, or the ECM itself. If you are unsure or uncomfortable performing electrical work, it’s advisable to seek professional help to avoid unintended damage or safety risks.
When to replace the ECM or seek professional service
In many cases, issues with P0685 are resolved by replacing a faulty relay, fixing damaged wiring, or replacing a blown fuse. However, if power delivery tests confirm the ECM is not receiving stable power despite a healthy relay and wiring, or if the ECM shows signs of internal failure, replacement or reprogramming may be required. A dealership or experienced automotive electrician can perform advanced diagnostics, reflash the PCM, or install a new module if needed.
Summary
P0685 on a 2011 Honda Pilot points to a fault in the ECM/PCM Power Relay Control Circuit. It most often stems from a bad power relay, blown fuse, wiring/connector problems, or a compromised ECM. A structured diagnostic approach—checking voltage, fuses, relays, and grounds—helps identify the root cause. Depending on the results, solutions range from replacing a relay or repairing wiring to ECM replacement or reprogramming, with professional service advisable for final resolution.
Final takeaways
If you encounter P0685, start with the electrical power path to the ECM and proceed methodically through relays, fuses, and connections. Keep safety in mind when handling the battery and electrical system, and consider professional diagnosis if the issue persists after basic repairs. A correct fix can restore starting reliability and prevent future electrical faults.
Summary of key steps
For quick reference, here are the essential steps to address P0685 on a 2011 Honda Pilot: verify DTCs, inspect battery/charging, check ECM power fuse and relay, test/swap the relay, examine connectors and grounds, inspect the wiring harness, and consider ECM replacement or reprogramming if power to the PCM cannot be established despite good relays and wiring.