P0685 — ECM/PCM Power Relay Control Circuit/Open #

Note: The definition of code P0685 may vary depending on the vehicle manufacturer (OEM).

• ECM/PCM Power Relay Control Circuit/Open (General/Standard)
• ECM Relay Control Circuit Malfunction (Honda, Acura)
• PCM Power Relay Control Circuit Malfunction (Ford, GM)
• ECM Main Relay Circuit/Open (Toyota, Lexus)

Summary of Code P0685 #

P0685 is a generic OBD-II trouble code that indicates the Engine Control Module (ECM) or Powertrain Control Module (PCM) has detected an issue with the relay that supplies power to it. In simple terms, the vehicle’s computer is not getting the electrical power it expects, or it cannot control the power relay as designed. This can lead to poor drivability, starting issues, or even a no-start condition. The code can be set due to wiring faults, a defective relay, or issues within the ECM/PCM itself. Timely diagnosis is important, as unresolved power supply problems can cause unpredictable engine behavior and potential stalling.

Common Symptoms for Code P0685 #

When P0685 is present, the vehicle can exhibit a range of symptoms, from subtle to severe. Because this code is related to the ECM/PCM’s main power supply, symptoms often appear suddenly and may prevent the engine from operating correctly. Watch for these signs:

• Engine will not start (cranks but does not fire)
• Intermittent stalling while driving
• Dash warning lights illuminated (Check Engine, etc.)
• No communication with scan tool (in severe cases)
• Rough running or misfires
• Electrical accessories may not function properly

Most Likely Causes for Code P0685 #

Several issues can trigger this code, but some causes are more common than others. Understanding the order of likelihood can save hours of diagnostic time:

1. Faulty ECM/PCM power relay — The relay itself is defective and cannot close the circuit.
2. Open or short in relay control wiring — Broken, corroded, or shorted wiring between the ECM/PCM and the relay.
3. Connector issues — Loose, corroded, or damaged connectors at the relay or the ECM/PCM.
4. Blown fuse in the circuit — A blown fuse prevents power delivery to the ECM/PCM.
5. Internal ECM/PCM failure — Rare, but possible; the control module itself is unable to control the relay.

Step-by-Step Diagnostic Procedure for Code P0685 #

Diagnosing P0685 requires patience and a methodical approach. Electrical issues can easily hide in plain sight, so take your time and check each step thoroughly:

1. Check for additional trouble codes — Other codes may help pinpoint the cause.
2. Inspect ECM/PCM power relay — Locate the relay (consult the owner’s manual or wiring diagram) and test it by swapping with a similar known-good relay, if available.
3. Test fuse(s) — Verify all related fuses are intact and have proper continuity.
4. Visually inspect wiring and connectors — Look for signs of corrosion, burns, or loose pins at the relay and ECM/PCM connectors.
5. Test relay control circuit — Using a multimeter, check for voltage at the relay’s control and load terminals per OEM specifications.
6. Test ground connections — Ensure all relevant ground points are clean, tight, and corrosion-free.
7. Check ECM/PCM operation — If all wiring and components test good, suspect an internal fault in the ECM/PCM (rare).

Possible Repairs for Code P0685 #

Once the root cause is identified, repairs are usually straightforward and targeted. Always follow OEM procedures and confirm the repair by clearing the code and testing the system:

• Replace faulty ECM/PCM power relay
• Repair or replace damaged wiring or connectors
• Replace blown fuse(s)
• Clean or secure ground points
• Replace or reprogram ECM/PCM (only if all other causes are ruled out)

Is It Safe to Drive with Code P0685? #

No, it is not recommended to drive with code P0685 active. This code points to a potentially critical electrical supply issue for your vehicle’s main computer. The engine may stall unexpectedly, fail to restart, or run erratically, which can pose a safety hazard. If you experience starting problems or stalling, have the vehicle inspected and repaired as soon as possible to avoid being stranded or causing further damage.

Vehicles Most Commonly Affected by Code P0685 #

P0685 is a generic code and can appear on almost any OBD-II compliant vehicle (1996-onward), but it is more frequently reported on certain models. Some vehicles with higher occurrences include:

• Honda Accord, Civic, CR-V
• Toyota Camry, Corolla, RAV4
• Ford F-150, Escape
• Chevrolet Silverado, Malibu
• Nissan Altima, Sentra
• Acura TL, MDX
• Chrysler/Dodge minivans and sedans

Frequent Mistakes & Best Practices for Code P0685 #

Diagnosing electrical relay codes can be tricky. To avoid common pitfalls and increase the success rate, keep these points in mind:

• Don’t replace the ECM/PCM without fully testing the relay and wiring — Internal control module failure is rare.
• Always check all related fuses, even those that seem unrelated — Sometimes, circuits are interconnected in unexpected ways.
• Visually inspect connectors for corrosion or bent pins — A simple cleaning can resolve many issues.
• Document your steps — Keeping track of what you’ve tested prevents going in circles.
• Consult OEM wiring diagrams — Each manufacturer can wire the relay circuit differently.

Quick FAQ for Code P0685 #

Still have questions? Here are answers to the most common concerns:

Q: Will clearing the code make the problem go away?
A: No, clearing P0685 without fixing the underlying issue will only make the code return. The root cause must be addressed for a lasting fix.

Q: Can I swap the relay with another one in the fuse box?
A: Often, yes. Many relays are identical. Swap with a relay of the same rating (such as the horn or A/C relay) to see if symptoms change.

Q: Is ECM/PCM failure common with this code?
A: Very rarely. Most P0685 cases are caused by a faulty relay, wiring, or fuse, not the control module itself.

If you’re unsure or stuck, don’t hesitate to consult a professional technician. Electrical diagnostics can be challenging, but a systematic approach will usually reveal the culprit.