P0685 Code: ECM/PCM Power Relay Control Circuit Open

When your vehicle’s diagnostic system detects trouble code P0685, it means the engine control module (ECM) or powertrain control module (PCM) has identified a problem with the power relay control circuit. This circuit is responsible for supplying power to the ECM/PCM itself—essentially, it’s the “on/off switch” for your vehicle’s main computer. When this circuit is open or malfunctioning, your engine may not start, run erratically, or shut down unexpectedly.

What Does P0685 Mean?

The P0685 diagnostic trouble code specifically refers to an open circuit in the ECM/PCM power relay control circuit. Here’s how it works:

• The Power Relay: This relay acts as an electrical switch controlled by the ignition system. When you turn the key to “On,” the relay energizes and supplies 12V power directly to the ECM/PCM.
• The Control Circuit: The ECM/PCM sends a signal to the relay to keep it energized while the engine is running. If this control signal path is broken (open), the relay cannot maintain power delivery.
• The Problem: When the circuit is open, the ECM/PCM loses power or cannot properly control the relay, resulting in insufficient power supply to the module itself.

This is a critical circuit because without proper power relay operation, the engine control module cannot function correctly, affecting engine starting, fuel injection, ignition timing, and emission controls.

Common Symptoms of P0685

• Check Engine Light illuminated on the dashboard
• Engine cranks but won’t start or starts intermittently
• Engine stalls while driving or at idle
• Loss of power steering (if electrically assisted)
• Fuel pump not running (no priming sound when key is turned on)
• Dashboard lights flickering or dimming
• Difficulty starting in cold weather
• Multiple fault codes appearing simultaneously
• Clicking sound from the relay area when trying to start
• Vehicle completely fails to start

Possible Causes (Ranked by Frequency)

1. Faulty Power Relay

The most common cause of P0685 is a failing or defective power relay. Relays wear out over time due to electrical arcing and heat exposure. A relay that cannot close its contacts properly or loses continuity will trigger this code. This is typically the first component to inspect.

2. Loose or Corroded Relay Connectors

Poor electrical connections at the relay socket or wiring harness connectors can cause an intermittent open circuit. Corrosion, moisture, or loose pins in the connector are frequent culprits. These connections are often located in the engine bay fuse/relay box where they’re exposed to heat and moisture.

3. Broken or Damaged Wiring

Wiring harness damage between the ignition switch, relay, and ECM/PCM can create an open circuit. Chafed insulation, pinched wires, or rodent damage in the engine bay may break the control signal path.

4. Defective Ignition Switch

The ignition switch controls the initial signal that energizes the power relay. A faulty ignition switch may not send the proper signal to activate the relay, resulting in this code.

5. Blown Fuse or Open Fusible Link

A blown fuse or open fusible link in the power relay circuit will create an open condition. Check the fuse box for any blown fuses related to the power relay circuit.

6. Defective ECM/PCM

In rare cases, the ECM/PCM itself may be faulty and unable to properly control the relay circuit. This is typically the last diagnosis after all other components have been tested.

7. Battery or Charging System Issues

Insufficient battery voltage or a failing alternator may prevent the relay from energizing properly, though this typically triggers different codes.

Diagnostic Steps

Step 1: Retrieve and Document the Code

Use an OBD-II scanner to confirm code P0685 and check for any additional fault codes. Document all codes present, as multiple codes can help pinpoint the root cause. Clear the code and test drive to see if it returns immediately (indicating an active fault) or if it was intermittent.

Step 2: Visual Inspection

Locate the power relay in your vehicle’s fuse/relay box (consult your owner’s manual or service manual for the exact location). Inspect for:

• Corrosion or green/white oxidation on relay pins
• Loose or cracked relay housing
• Burnt or discolored contacts
• Damaged wiring near the relay
• Moisture inside the relay socket

Step 3: Check Relay Connectors

Unplug the power relay from its socket. Inspect the relay pins and socket contacts for corrosion, damage, or loose fit. Clean any corrosion with electrical contact cleaner. Reinsert the relay firmly until it clicks into place. Test the vehicle to see if the code clears.

Step 4: Test the Relay

Perform a relay click test: Turn the ignition to “On” and listen for a clicking sound from the relay. If you hear a click, the relay is energizing. If there’s no click, the relay may be faulty or not receiving the control signal. You can also swap the power relay with an identical relay from another circuit (like a fuel pump relay) to test if the problem follows the relay.

Step 5: Check Wiring and Connectors

Inspect all wiring harnesses connected to the power relay for damage, pinching, or corrosion. Check connector pins for proper seating and corrosion. Use a multimeter to test for continuity along the control circuit wiring. Compare resistance readings to your vehicle’s service manual specifications.

Step 6: Test Battery Voltage

Measure battery voltage at the relay with the ignition off (should be 12V+) and with the ignition on (should remain 12V+). Low voltage may prevent the relay from functioning properly.

Step 7: Check the Ignition Switch

Test the ignition switch output with a multimeter. The switch should provide a signal to the relay when turned to the “On” position. If no signal is present, the ignition switch may be faulty.

Step 8: Professional Diagnosis

If the above steps don’t resolve the issue, use a professional diagnostic scanner to monitor the relay control signal in real-time. This will show whether the ECM/PCM is properly commanding the relay and whether the relay is responding as expected.

Repair Cost Estimates

Note: Costs vary significantly by vehicle make, model, year, and location. Always get a quote from your mechanic before authorizing repairs.

Can I Still Drive?

Severity: Moderate to High

Whether you can safely drive with code P0685 depends on the severity of the problem:

• If the engine won’t start: You cannot drive the vehicle at all. The power relay is essential for engine operation.
• If the engine starts but stalls: Do not drive the vehicle. An engine that stalls unexpectedly is a serious safety hazard, especially in traffic or at highway speeds.
• If the code is intermittent: The vehicle may start and run normally most of the time, but you risk sudden stalling. Avoid highway driving and have the vehicle diagnosed and repaired as soon as possible.

Safety Recommendation: Code P0685 indicates a critical power supply issue to your engine control module. Do not ignore this code. Have the vehicle diagnosed and repaired by a qualified mechanic before extended driving. Failure to address this issue can result in sudden engine shutdown, loss of power steering, and potential accidents.

Frequently Asked Questions

Q: Can a bad battery cause code P0685?

A: While a weak battery can cause starting issues, it typically won’t directly trigger code P0685. However, low battery voltage can prevent the relay from energizing properly. If your battery is old (3+ years), have it tested. That said, if the battery tests good but the code persists, the problem is likely the relay, wiring, or ignition switch.

Q: Is code P0685 the same as a no-start condition?

A: Not necessarily. Code P0685 indicates a problem with the power relay control circuit, which can cause a no-start condition, but the vehicle might also start and then stall, or start intermittently. The code is specifically about the relay control circuit being open, not just the inability to start.

Q: Can I replace the power relay myself?

A: Yes, if you have basic mechanical skills. The power relay is typically located in the engine bay fuse/relay box and can be unplugged and replaced in minutes. However, if the problem is wiring or the ignition switch, professional diagnosis is recommended. Always consult your vehicle’s service manual for the correct relay location and specifications.

Q: What’s the difference between P0685 and P0686?

A: P0685 indicates an open circuit in the ECM/PCM power relay control circuit, while P0686 indicates a short circuit in the same circuit. P0686 suggests the relay is being over-energized or shorted to ground, whereas P0685 means the circuit is broken or not completing properly.