The P0685 diagnostic trouble code (DTC) is a generic powertrain code that indicates an issue with the Engine Control Module (ECM) or Powertrain Control Module (PCM) power relay control circuit. This code is typically set when the vehicle’s computer detects a low or no-voltage condition in the circuit responsible for supplying battery voltage to the ECM/PCM. Understanding and resolving this code is crucial for maintaining proper vehicle operation and preventing potential drivability issues.
| P0685 Code Meaning | P0685 Code Common Causes |
|---|---|
| ECM/PCM Power Relay Control Circuit Open | Faulty PCM power relay |
| Low or no-voltage condition detected in PCM supply circuit | Corroded or damaged wiring |
| Malfunction in the power supply to the engine computer | Blown fuse or fusible link |
| Interruption in the PCM’s power source | Defective ignition switch |
| Failure in the circuit that controls PCM power relay | Loose or corroded battery cable ends |
Symptoms of P0685 Code
When a vehicle experiences the P0685 code, several symptoms may manifest, alerting the driver to a potential issue with the ECM/PCM power relay control circuit. Recognizing these symptoms is crucial for prompt diagnosis and repair.
- Engine No-Start Condition: The most common and noticeable symptom is the vehicle’s inability to start. Since the PCM requires proper power to function, a failure in its power supply can prevent the engine from starting altogether.
- Hard Starting: In some cases, the engine may start but with significant difficulty, especially in cold weather conditions.
- Check Engine Light Illumination: The check engine light will typically illuminate on the dashboard when the P0685 code is set.
- Intermittent Power Loss: Drivers may experience sudden loss of power while driving, with the engine potentially stalling or running roughly.
- Electrical System Malfunctions: Various electrical components controlled by the PCM may exhibit erratic behavior or cease to function properly.
- Poor Fuel Economy: If the PCM is not receiving consistent power, it may not be able to optimize fuel injection, leading to decreased fuel efficiency.
- Erratic Idle: The engine may idle roughly or inconsistently due to improper PCM function.
- Transmission Issues: In vehicles with electronically controlled transmissions, shifting problems may occur if the PCM is not receiving adequate power.
It’s important to note that these symptoms can vary depending on the vehicle make and model, as well as the severity of the circuit issue.
Technical Explanation of P0685 Code
The P0685 code is specifically related to the power supply circuit of the Engine Control Module (ECM) or Powertrain Control Module (PCM). To understand this code, it’s essential to grasp how the PCM power relay system functions in modern vehicles.
In most OBD-II equipped vehicles, a relay is used to supply the PCM with battery voltage. This relay, often referred to as the PCM power relay, is typically a five-terminal device. Here’s a breakdown of its operation:
- Primary Input Terminal: Receives a constant supply of battery voltage.
- Ground Terminal: Connected to the engine or chassis ground.
- Secondary Input Terminal: Receives battery voltage through a fused circuit when the ignition switch is in the “ON” position.
- Output Terminal: Supplies power to the PCM.
- Signal Wire Terminal: Connected to the Controller Area Network (CAN) for communication.
When the ignition switch is turned to the “ON” position, it energizes a small coil inside the relay. This action closes the contacts within the relay, completing the circuit and allowing battery voltage to flow to the PCM through the output terminal.
The P0685 code is set when the PCM detects an abnormality in this power supply circuit. Specifically, it indicates that the voltage in the relay coil control circuit is higher than expected – typically above 4.6 volts with the key in the “ON” position. This high voltage suggests an open circuit, meaning there’s a break or interruption in the electrical path.
The PCM continuously monitors this circuit, and if it detects a low or no-voltage condition when it expects power, it will set the P0685 code. This monitoring is crucial because the PCM requires a stable and consistent power supply to function correctly and control various vehicle systems effectively.
Step-by-Step Diagnosis of P0685 Code
Diagnosing the P0685 code requires a systematic approach to identify the root cause of the ECM/PCM power relay control circuit issue. Here’s a comprehensive step-by-step guide for mechanics and advanced DIY enthusiasts:
- Preliminary Steps:
- Ensure the vehicle’s battery is fully charged and in good condition.
- Verify that there are no loose or corroded battery terminals.
- Check for any visible damage to wiring harnesses, especially near the PCM and fuse box.
- Scan for Additional Codes:
- Use an OBD-II scanner to check for any other stored codes.
- Clear all codes and see if P0685 returns after a short drive cycle.
- Inspect Fuses and Fusible Links:
- Locate the fuse for the PCM power circuit in the fuse box (refer to the vehicle’s manual).
- Visually inspect the fuse and replace if blown.
- Check any fusible links in the PCM power circuit for continuity.
- Examine the PCM Power Relay:
- Locate the PCM power relay (usually in the under-hood fuse box).
- Remove the relay and inspect for any signs of damage or corrosion.
- Test the relay’s functionality using a multimeter or by swapping with a known good relay.
- Check Wiring and Connections:
- Inspect all wiring and connectors in the PCM power circuit for damage, corrosion, or loose connections.
- Pay special attention to ground connections and the wiring near the PCM.
- Perform a continuity test on suspicious wires.
- Test the Ignition Switch:
- Check the ignition switch for proper operation using a multimeter.
- Ensure it’s providing the correct voltage to the PCM relay when in the “ON” position.
- Voltage Drop Test:
- Perform a voltage drop test across the PCM power circuit to identify any high-resistance points.
- Check for voltage drops at connections, fuses, and the relay.
- PCM Input Voltage Check:
- With the ignition on, measure the voltage at the PCM power input pin.
- Compare the reading to the battery voltage – it should be within 0.5 volts.
- PCM Ground Circuit Test:
- Check the resistance between the PCM ground terminals and the battery negative terminal.
- The resistance should be very low (less than 0.5 ohms).
- Advanced Diagnostics:
- If all previous tests pass, consider using an oscilloscope to check for any abnormal voltage fluctuations in the PCM power circuit.
- Consult manufacturer-specific diagnostic procedures for any additional tests.
Remember, if at any point you feel uncomfortable or unsure about performing these diagnostic steps, it’s best to consult a professional automotive technician.
Solution Methods for P0685 Code
After diagnosing the P0685 code, the next step is to implement the appropriate solution based on the identified issue. Here are detailed solution methods for common causes of the P0685 code:
- Replacing the PCM Power Relay:
- Locate the PCM power relay in the fuse box.
- Remove the old relay and replace it with a new, compatible relay.
- Ensure the new relay is securely seated in its socket.
- Test the vehicle to confirm the code has been cleared.
- Repairing or Replacing Damaged Wiring:
- Identify the damaged section of wiring in the PCM power circuit.
- If the damage is minor, repair the wire using appropriate gauge wire and heat-shrink tubing.
- For extensive damage, replace the entire wiring harness section.
- Secure all connections and use dielectric grease to prevent future corrosion.
- Replacing Blown Fuses or Fusible Links:
- Remove the blown fuse or fusible link.
- Replace with a new component of the same amperage rating.
- Investigate the cause of the blown fuse to prevent recurrence.
- Cleaning or Replacing Corroded Connections:
- Clean corroded terminals with a wire brush and electrical contact cleaner.
- If corrosion is severe, cut back the wire to clean copper and install new terminals.
- Apply dielectric grease to prevent future corrosion.
- Repairing or Replacing the Ignition Switch:
- Remove the steering column covers to access the ignition switch.
- Disconnect the electrical connectors and remove the switch.
- Install a new ignition switch and reconnect all electrical connections.
- Test for proper operation before reassembling the steering column.
- Addressing PCM Ground Issues:
- Locate all ground points for the PCM circuit.
- Clean ground connections with a wire brush to ensure good contact.
- If necessary, relocate ground points to clean, unpainted metal surfaces.
- Secure all ground connections tightly.
- PCM Replacement (as a last resort):
- Confirm that all other potential causes have been ruled out.
- Obtain a new or remanufactured PCM specific to your vehicle model.
- Have the new PCM programmed with your vehicle’s VIN and security codes.
- Install the new PCM and perform necessary relearn procedures.
Warning: PCM replacement should only be done after exhausting all other options and should be performed by a qualified technician to ensure proper programming and installation.
After implementing any of these solutions, clear the diagnostic trouble codes and perform a test drive to ensure the P0685 code does not return. If the code persists, further diagnosis may be necessary.
Cost Estimates for P0685 Code Repairs
The cost of repairing issues related to the P0685 code can vary significantly depending on the root cause and the vehicle make and model. Here’s a breakdown of potential costs:
- Diagnostic Fee: $75 – $150
Most repair shops charge a diagnostic fee to pinpoint the exact cause of the P0685 code. - PCM Power Relay Replacement: $50 – $200
- Parts: $20 – $100
- Labor: $30 – $100
- Wiring Repair: $100 – $500+
Costs can vary widely depending on the extent of the damage and the complexity of the repair. - Fuse or Fusible Link Replacement: $20 – $100
- Parts: $5 – $30
- Labor: $15 – $70
- Ignition Switch Replacement: $150 – $400
- Parts: $50 – $200
- Labor: $100 – $200
- PCM Replacement: $500 – $2,000+
- Parts: $300 – $1,500
- Labor: $200 – $500
- Programming: $100 – $200
- Battery Cable Replacement: $150 – $400
- Parts: $50 – $200
- Labor: $100 – $200
These estimates are general ranges, and actual costs may fall outside these ranges depending on your specific vehicle and location. Always obtain multiple quotes from reputable repair shops for the most accurate pricing.
Note: DIY repairs can significantly reduce costs but should only be attempted if you have the necessary skills and tools. Improper repairs can lead to more expensive damage or safety issues.
Warnings and Recommendations
When dealing with the P0685 code and its related repairs, keep the following warnings and recommendations in mind:
- Safety First: Always disconnect the battery before working on electrical systems to prevent short circuits or electrical shocks.
- Use Proper Tools: Ensure you have the correct diagnostic tools and repair equipment before attempting any fixes.
- Avoid Quick Fixes: Temporary solutions like bypassing relays or fuses can lead to more severe electrical issues or safety hazards.
- Protect Sensitive Electronics: Use anti-static precautions when handling the PCM or other electronic components to prevent damage from static electricity.
- Follow Manufacturer Guidelines: Always consult your vehicle’s service manual for specific diagnostic procedures and repair instructions.
- Quality Parts: Use OEM or high-quality aftermarket parts to ensure reliability and prevent recurring issues.
- Professional Help: If you’re unsure about any aspect of the diagnosis or repair, seek help from a certified automotive technician.
- Post-Repair Testing: After making repairs, thoroughly test the vehicle to ensure the P0685 code doesn’t return and that all systems are functioning correctly.
- Address Related Issues: If other codes are present along with P0685, address them as well, as they may be interconnected.
- Regular Maintenance: Keep up with routine vehicle maintenance to prevent electrical issues that could lead to codes like P0685.
Warning: Misdiagnosis or improper repairs can lead to more severe electrical issues, potentially damaging expensive components like the PCM. If you’re not confident in your ability to diagnose and repair the issue, it’s best to consult a professional.
The P0685 code, while often indicative of a relatively straightforward electrical issue, can sometimes be a symptom of more complex problems within your vehicle’s electrical system. By following the diagnostic steps outlined in this guide and adhering to the warnings and recommendations provided, you can effectively troubleshoot and resolve the issue, ensuring your vehicle’s engine management system functions optimally.
Remember that maintaining your vehicle’s electrical system is crucial for overall performance and longevity. Regular inspections of wiring, connections, and battery condition can help prevent issues like the P0685 code from occurring in the first place. If you encounter persistent problems or feel uncomfortable with any aspect of the diagnosis or repair process, don’t hesitate to seek the expertise of a qualified automotive technician.
By addressing the P0685 code promptly and correctly, you can avoid more serious and costly repairs down the road, ensuring your vehicle remains reliable and efficient for years to come.
Frequently Asked Questions About P0685
Repair time can vary from 1-4 hours,
- Can I drive my car with the P0685 code?
It’s not recommended to drive with this code present. The vehicle may not start or could stall while driving, posing a safety risk. - How urgent is it to fix the P0685 code?
Very urgent. This code affects the PCM’s power supply, which is crucial for proper engine operation. Immediate diagnosis and repair are necessary. - Will the P0685 code clear itself?
No, this code typically won’t clear on its own. It requires diagnosing and fixing the underlying issue, then clearing the code with a scanner. - Can a bad battery cause the P0685 code?
Yes, a weak or failing battery can sometimes trigger this code. Always check the battery condition as part of your diagnosis. - How long does it take to fix a P0685 code?
Repair time can vary from 1-4 hours,Contents +