The P0685 trouble code is a diagnostic trouble code (DTC) that indicates an issue with the ECM/PCM (Engine Control Module/Powertrain Control Module) power relay control circuit. This code is generated when the PCM detects a low or no-voltage condition in the circuit that supplies power to the PCM. It is applicable to all OBD-II compliant vehicles manufactured since 1996, making it a common issue among various makes and models. Understanding this code is crucial for vehicle owners and DIY mechanics, as it can lead to significant engine performance problems if left unaddressed.
| P0685 Code Meaning | P0685 Code Common Causes |
|---|---|
| ECM/PCM Power Relay Control Circuit Open | Faulty PCM power relay |
| Low or no voltage detected in the circuit | Blown fuse or fusible link |
| Powertrain control module malfunction | Corroded or damaged wiring/connectors |
| Electrical connection issues | Faulty ignition switch |
| Intermittent electrical faults | Loose or corroded battery cables |
Symptoms of P0685 Code
When the P0685 code is present, you may experience several symptoms that can affect your vehicle’s operation:
- Check Engine Light: The most common symptom is the illumination of the check engine light on your dashboard.
- Starting Issues: The vehicle may exhibit hard starting or may not start at all, especially in cold weather conditions.
- Engine Performance Problems: You may notice reduced engine power, stalling, or irregular acceleration.
- Electrical Malfunctions: Other electrical components, such as lights and radio, may behave erratically due to power supply issues.
Technical Explanation of P0685 Code
The P0685 code indicates that the PCM has detected a problem with the power relay circuit that supplies battery voltage to it. The power relay plays a critical role in ensuring that the PCM receives adequate voltage to function correctly. When this circuit is open (i.e., not completing), it can lead to various operational issues.
The relay typically has five terminals:
- Input Terminal: Receives constant battery voltage.
- Ground Terminal: Connected to engine or chassis ground.
- Secondary Input Terminal: Receives voltage when the ignition switch is in the “ON” position.
- Output Terminal: Provides battery voltage to the PCM.
- Signal Wire: Communicates with other modules in the vehicle.
When you turn on the ignition, a small coil inside the relay energizes, closing contacts and allowing current to flow to the PCM. If there’s an issue—like a blown fuse, corroded wiring, or a faulty relay—the PCM will set the P0685 code.
Step-by-Step Diagnosis of P0685 Code
Step 1: Initial Scan
- Use an OBD-II scanner to read stored codes and freeze frame data.
- Record any additional codes present; these may provide clues about related issues.
Step 2: Visual Inspection
- Inspect battery cables for corrosion or loose connections.
- Check all relevant wiring harnesses for damage or disconnections near the PCM and relay.
Step 3: Test Power Relay
- Remove the power relay from its socket.
- Using a multimeter, check for continuity across the relay terminals.
- If continuity is absent, replace the relay.
Step 4: Check Voltage Supply
- With the ignition in the “ON” position, measure voltage at the input terminal of the relay.
- If voltage is absent, check for blown fuses or issues with the ignition switch.
Step 5: Inspect Ground Connections
- Ensure that ground connections are secure and free of corrosion.
- Measure resistance between ground points and ensure they are within acceptable limits.
Step 6: Test PCM Output
- Check for voltage at the output terminal of the relay when energized.
- If no voltage is present at this stage but everything else checks out, suspect a faulty PCM.
Solution Methods for P0685 Code
Replace Faulty Components
- Power Relay Replacement: If testing indicates a faulty relay, replace it with a new one.
- Repair Wiring Issues: Fix any damaged wires or connectors found during inspection.
- Fuse Replacement: Replace any blown fuses that may be interrupting power supply.
Professional Diagnosis
If you are unable to identify or resolve the issue after following these steps, it may be wise to consult with a professional mechanic who can perform more advanced diagnostics using specialized equipment.
Cost Estimates for Repairs
The cost of repairs associated with P0685 can vary widely based on several factors:
- DIY Repairs: If you perform repairs yourself, costs may include parts such as relays ($20-$50), fuses ($5-$15), and wiring materials ($10-$30).
- Professional Repairs: Taking your vehicle to a mechanic could range from $100 to $300 depending on labor rates and parts needed. Complex issues involving PCM replacement can exceed $1,000.
Warnings and Recommendations
- Always disconnect your vehicle’s battery before working on electrical components to avoid shorts or further damage.
- If you’re not confident in your ability to diagnose or repair electrical issues, seek professional help to prevent costly mistakes.
- Regular maintenance checks on battery connections and wiring can help prevent issues related to DTCs like P0685 from occurring in the first place.
Closing Paragraph
Understanding and addressing DTCs like P0685 is essential for maintaining your vehicle’s performance and reliability. Whether you’re a seasoned DIY mechanic or a vehicle owner looking to troubleshoot problems, knowing how to diagnose and repair these codes can save time and money. Always prioritize safety and consider professional assistance when dealing with complex electrical systems in your vehicle.
Frequently Asked Questions About P0685
- What does code P0685 mean?
The P0685 code indicates an issue with the ECM/PCM power relay control circuit, specifically that there is an open circuit preventing proper voltage supply. - What are common symptoms of P0685?
Common symptoms include check engine light illumination, starting difficulties, reduced engine performance, and potential electrical malfunctions. - How serious is code P0685?
This code can lead to significant engine performance issues and should be addressed promptly to avoid further complications. - Can I drive my car with a P0685 code?
While you may be able to drive your vehicle temporarily, it is not advisable as this could lead to stalling or complete failure. - How do I fix code P0685?
Fixing this code typically involves inspecting and replacing faulty relays, repairing wiring issues, or replacing blown fuses. - Is it safe to replace components myself?
If you have experience with automotive repairs, you can replace components yourself; otherwise, consult a professional. - What tools do I need for diagnosis?
A basic OBD-II scanner and multimeter are essential tools for diagnosing this trouble code. - How much will it cost to fix code P0685?
The cost can range from $100 for simple fixes up to $1,000 for complex repairs involving component replacements.