P0685 Diagnostic Trouble Code: Complete Expert Guide
Everything You Need to Know About ECM/PCM Power Relay Control Circuit Issues
Understanding P0685 Code
The P0685 diagnostic trouble code (DTC) indicates a malfunction in the engine control module (ECM) or powertrain control module (PCM) power relay control circuit. This critical circuit supplies power to the vehicle’s main computer, and any disruption can cause significant drivability issues.
Technical Definition
P0685 is set when the ECM/PCM detects an irregular voltage or resistance in the power relay control circuit. This circuit typically includes:
- ECM/PCM Power Relay: The main relay that supplies power to the engine computer
- Control Circuit Wiring: The wiring harness connecting the relay to the ECM/PCM
- Power Supply Circuit: The main power feed from the battery to the relay
- Ground Circuits: The grounding paths for the relay and ECM/PCM
Common Names for P0685
OEM Terminology
- ECM/PCM Power Relay Control Circuit/Open
- Engine Control Module Power Supply Relay Circuit Malfunction
- PCM Power Relay Control Circuit
- ECU Main Relay Circuit Malfunction
Common Descriptions
- Engine Computer Power Relay Problem
- Main Relay Circuit Issue
- ECM Power Supply Fault
- Computer Relay Control Circuit Open
Symptoms of P0685 Code
When the P0685 code is stored, you may experience one or more of the following symptoms, ranging from mild inconvenience to complete vehicle disablement:
Primary Symptoms
- Check Engine Light: The most common and obvious symptom, often the only initial sign
- Engine No-Start Condition: The vehicle may not start or may be difficult to start, especially when hot
- Engine Stalling: The engine may stall while driving or at idle, often without warning
- Reduced Engine Performance: Lack of power, hesitation, or rough running under load
- Intermittent Issues: Problems that come and go unpredictably, making diagnosis challenging
Secondary Symptoms
- Other Warning Lights: Additional warning lights may illuminate on the dashboard
- Transmission Issues: In vehicles with electronically controlled transmissions, shifting problems may occur
- Electrical Gremlins: Various electrical components may malfunction intermittently
- Battery Drain: In some cases, the faulty circuit can cause parasitic battery drain
Diagnosing P0685 Code
Diagnosing a P0685 code requires a systematic approach using proper diagnostic tools and procedures. Follow these steps for accurate diagnosis:
Required Diagnostic Tools
Step-by-Step Diagnostic Procedure
- Scan for Codes: Use an OBD-II scanner to confirm P0685 is present and check for any additional codes that might provide clues about related issues.
- Visual Inspection: Thoroughly inspect all related fuses, relays, and wiring for obvious damage, corrosion, or loose connections. Pay special attention to the main engine bay fuse box.
-
Test the ECM/PCM Power Relay:
- Locate the ECM/PCM power relay (consult service manual for location)
- Test the relay using a multimeter or relay tester
- Check for proper voltage at the relay terminals with key on and during cranking
- Verify the relay is properly engaging when energized
- Check Power Supply: Verify that the ECM/PCM is receiving proper voltage and ground. Check both battery voltage and ignition-switched power sources.
- Inspect Wiring: Check all wiring between the relay, ECM/PCM, and battery for damage, shorts, or opens. Pay attention to areas where wiring may rub against sharp edges.
- ECM/PCM Testing: If all other components check out, the ECM/PCM itself may be faulty and require testing or replacement. This should be a last resort after all other possibilities are eliminated.
Advanced Diagnostic Tests
| Test | Procedure | Expected Result |
|---|---|---|
| Relay Control Circuit Voltage | Measure voltage at relay control terminal with key ON | Battery voltage (12V) |
| Relay Power Circuit Voltage | Measure voltage at relay power terminals | Constant battery voltage |
| ECM/PCM Power Supply | Check voltage at ECM/PCM power pins | Battery voltage with key ON |
| Circuit Resistance | Measure resistance of control circuit wiring | Less than 5 ohms |
| Voltage Drop Test | Test voltage drop across power and ground circuits | Less than 0.5V |
Repair Procedures for P0685
Once the cause of the P0685 code has been identified through proper diagnosis, follow these repair procedures based on the specific issue found:
Common Repairs for P0685
| Component | Repair Procedure | Difficulty | Estimated Time |
|---|---|---|---|
| ECM/PCM Power Relay | Replace the faulty relay with an OEM or high-quality replacement | Easy | 15-30 minutes |
| Blown Fuse | Replace the fuse and investigate what caused it to blow | Easy | 5-15 minutes |
| Wiring Issues | Repair or replace damaged wiring, connectors, or terminals | Moderate | 1-3 hours |
| Faulty ECM/PCM | Replace or reprogram the engine control module | Difficult | 2-4 hours + programming |
| Bad Ground Connection | Clean and secure ground connections to the chassis or engine | Moderate | 30-60 minutes |
| Ignition Switch Issues | Replace faulty ignition switch if not providing proper power | Moderate | 1-2 hours |
Detailed Repair Instructions
Relay Replacement
- Locate the ECM/PCM power relay (consult vehicle service manual)
- Disconnect the negative battery terminal for safety
- Remove the old relay from its socket
- Install the new relay, ensuring proper orientation
- Reconnect battery and test operation
- Clear codes and verify repair
Wiring Repair
- Identify damaged section of wiring using wiring diagrams
- Disconnect battery and isolate the damaged circuit
- Repair or replace the damaged section using proper techniques
- Protect repaired area with loom and secure properly
- Reconnect battery and test circuit operation
- Clear codes and verify repair
Repair Cost Estimates
The cost to repair a P0685 code can vary significantly based on the root cause, your vehicle make/model, and labor rates in your area:
Cost Breakdown by Repair Type
| Repair | Parts Cost | Labor Cost | Total Estimate | Warranty |
|---|---|---|---|---|
| Relay Replacement | $15 – $50 | $50 – $100 | $65 – $150 | 1 year |
| Fuse Replacement | $5 – $20 | $30 – $60 | $35 – $80 | 90 days |
| Wiring Repair | $20 – $100 | $100 – $300 | $120 – $400 | 1 year |
| ECM/PCM Replacement | $500 – $1500 | $200 – $500 | $700 – $2000 | 2-3 years |
| Ignition Switch Replacement | $80 – $200 | $150 – $300 | $230 – $500 | 1 year |
Cost by Vehicle Type
| Vehicle Type | Average Diagnostic Cost | Average Repair Cost | Common Issues |
|---|---|---|---|
| Economy Cars | $80 – $120 | $100 – $400 | Relay failure, wiring issues |
| Luxury Vehicles | $120 – $200 | $300 – $1500 | ECM failure, complex wiring |
| Trucks & SUVs | $90 – $150 | $150 – $800 | Ground issues, relay failure |
| European Imports | $100 – $180 | $200 – $1200 | ECM issues, specialized relays |
Note: These are estimates only. Actual costs may vary based on your location, vehicle, and shop rates. Always get a detailed estimate before authorizing repairs.
Prevention & Maintenance
To prevent P0685 and similar electrical issues, follow these maintenance practices:
Preventive Maintenance Schedule
| Maintenance Item | Frequency | Procedure |
|---|---|---|
| Battery Terminal Cleaning | Every 6 months | Clean terminals and apply protective coating |
| Fuse Box Inspection | Annually | Check for corrosion, loose fuses, damaged covers |
| Relay Inspection | Every 2 years or 30,000 miles | Test critical relays including ECM power relay |
| Wiring Harness Inspection | Every 2 years | Check for chafing, damage, or loose connections |
| Ground Connection Check | Annually | Clean and tighten main engine and chassis grounds |
Additional Prevention Tips
- Address electrical issues promptly to prevent further damage to sensitive components
- Use OEM or high-quality replacement parts for all electrical repairs
- Keep the engine compartment clean and free of excessive moisture
- Follow the manufacturer’s recommended maintenance schedule
- Avoid aftermarket electrical accessories that may overload circuits
- Have charging system checked regularly to ensure proper voltage regulation
Frequently Asked Questions
It depends on the symptoms. If the vehicle is running normally with only the check engine light illuminated, short trips may be possible. However, if you’re experiencing stalling, hard starting, or reduced performance, it’s not safe to drive as the vehicle could stall unexpectedly. Have the issue diagnosed as soon as possible.
These codes are all related to the ECM/PCM power relay circuit but indicate different specific issues:
- P0685: General circuit malfunction or open circuit
- P0686: Control circuit voltage too low
- P0687: Control circuit voltage too high
Diagnosis time varies based on the specific cause and technician experience. Simple issues like a bad relay or fuse may be diagnosed in 30-60 minutes. More complex wiring problems or intermittent issues can take 2-4 hours or more to properly diagnose. ECM/PCM diagnosis typically takes 1-2 hours before determining if replacement is necessary.
While a weak or failing battery typically doesn’t directly cause a P0685 code, it can contribute to electrical issues that might set this code. Low system voltage can prevent relays from functioning properly or cause ECM/PCM reset issues. Always ensure your battery and charging system are in good condition when diagnosing electrical codes.
Coverage depends on your vehicle’s warranty status. New vehicles typically cover ECM/PCM issues under powertrain warranty. For older vehicles, it depends on the specific warranty terms. Aftermarket extended warranties may cover this repair depending on the cause. Always check your warranty documentation and consult with your dealer or warranty provider.
