P0685 Code: Here's How to Solve It Fast

The P0685 code is a diagnostic trouble code (DTC) 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 addressing this code promptly is crucial for maintaining your vehicle's performance and preventing potential damage to critical components.

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
Interruption in power supply to the ECM/PCM	Blown fuse or fusible link
Malfunction in the PCM relay control system	Faulty ignition switch
Potential PCM programming error	Loose or corroded battery cable ends

Symptoms of P0685 Code

When your vehicle's computer sets the P0685 code, you may experience several symptoms that can affect your driving experience and vehicle performance. It's essential to be aware of these signs to address the issue promptly:

• Illuminated Check Engine Light
• Engine hard starts or fails to start
• Engine stalling
• Reduced throttle response and acceleration
• Engine entering limp mode
• Intermittent loss of electrical power at idle
• Tachometer and speedometer dropping to zero
• Radio or other electrical components shutting off unexpectedly

These symptoms can vary depending on the severity of the issue and the specific vehicle make and model. Some drivers may experience all of these symptoms, while others might only notice one or two. Regardless of the severity, it's crucial to address the P0685 code as soon as possible to prevent further damage to your vehicle's electrical system and engine components.

Technical Explanation of P0685 Code

To understand the P0685 code fully, it's important to delve into the technical aspects of how the ECM/PCM power relay control circuit functions. The Powertrain Control Module (PCM) is the brain of your vehicle's engine management system, responsible for controlling various aspects of engine operation, including fuel injection, ignition timing, and emissions control.

The PCM requires a constant and stable power supply to function correctly. This power is typically provided through a relay, which acts as a switch to control the flow of electrical current to the PCM. The relay is usually a five-terminal design with the following connections:

• A constant supply of battery voltage to the primary input terminal
• A ground terminal connected to the engine or chassis ground
• A secondary input terminal that receives battery voltage when the ignition switch is in the "ON" position
• An output terminal that supplies power to the PCM
• A signal wire for the Controller Area Network (CAN)

When you turn the ignition key 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. The PCM continuously monitors the voltage in this circuit, expecting to see the correct voltage levels at the appropriate times.

The P0685 code is set when the PCM detects an abnormality in this power supply circuit. Specifically, it indicates that the PCM has detected a low-voltage or no-voltage condition when it expects to see full battery voltage. This could be due to various issues, including a faulty relay, damaged wiring, or problems with the PCM itself.

Step-by-Step Diagnosis of P0685 Code

Diagnosing the P0685 code requires a systematic approach to identify the root cause of the issue. Follow these steps to diagnose the problem effectively:

1. Connect an OBD-II scanner to the vehicle's diagnostic port and retrieve all stored codes and freeze frame data. This information can provide valuable insights into the conditions present when the code was set.
2. Visually inspect the battery cables and connections for signs of corrosion, damage, or loose connections. Clean or tighten as necessary.
3. Check the fuses related to the PCM and its power supply. Replace any blown fuses, but be aware that a blown fuse is often a symptom of a larger problem.
4. Locate the PCM power relay (refer to your vehicle's service manual for the exact location) and inspect it for signs of damage or overheating.
5. Use a multimeter to test the PCM power relay:
   • With the ignition off, check for battery voltage at the relay's input terminal
   • Turn the ignition on and check for voltage at the relay's output terminal
   • If no voltage is present at the output with the ignition on, the relay may be faulty
6. Inspect the wiring harness connected to the PCM and the power relay for any signs of damage, fraying, or exposed wires.
7. Use a multimeter to check for continuity in the wiring between the relay and the PCM. Any breaks in continuity indicate a wiring issue that needs to be addressed.
8. If all previous steps check out, consider testing the PCM itself. This typically requires specialized equipment and expertise, so it may be best to consult a professional at this point.
9. After making any repairs or replacements, clear the codes and test drive the vehicle to ensure the P0685 code does not return.

Warning: Always disconnect the battery before working on electrical components to avoid the risk of shock or damage to sensitive electronics.

Solutions for P0685 Code

Once you've diagnosed the issue causing the P0685 code, you can proceed with the appropriate solution. Here are some common fixes based on the potential causes:

1. Replacing the PCM Power Relay

If the relay is found to be faulty during testing, replacing it is often the most straightforward solution. Ensure you use a relay that matches the specifications of your vehicle.

Cost estimate: $20 - $100 for the relay, plus labor if not done yourself.

2. Repairing or Replacing Wiring

For issues related to damaged or corroded wiring:

• Clean corrosion from connectors using electrical contact cleaner
• Repair any frayed or exposed wires using proper automotive wire and heat-shrink tubing
• Replace severely damaged sections of the wiring harness

Cost estimate: $50 - $300, depending on the extent of the damage and whether you perform the repairs yourself.

3. Replacing Fuses or Fusible Links

If a blown fuse is identified:

• Replace the fuse with one of the same amperage rating
• Investigate the cause of the blown fuse to prevent recurrence

Cost estimate: $5 - $20 for fuses, potentially more if the underlying issue requires additional repairs.

4. Repairing or Replacing the Ignition Switch

A faulty ignition switch can cause intermittent power issues:

• Test the ignition switch using a multimeter
• Replace the switch if it's not functioning correctly

Cost estimate: $150 - $400, including parts and labor.

5. PCM Replacement or Reprogramming

In rare cases, the PCM itself may be faulty or require reprogramming:

• PCM replacement should be considered a last resort after all other possibilities have been exhausted
• Reprogramming may be necessary if a software issue is identified

Cost estimate: $500 - $2000 for PCM replacement, $100 - $200 for reprogramming.

Warning: PCM replacement or reprogramming should only be performed by qualified professionals with the proper diagnostic and programming equipment.

Preventing Future P0685 Codes

To minimize the risk of encountering the P0685 code in the future, consider the following preventive measures:

• Regularly inspect and clean battery terminals and connections
• Perform routine electrical system checks as part of your vehicle maintenance
• Address any electrical issues promptly to prevent cascading failures
• Use high-quality replacement parts when repairs are necessary
• Avoid exposing your vehicle's electrical system to extreme conditions (e.g., water damage, excessive heat)

By following these preventive measures and addressing any P0685 codes promptly, you can help ensure the longevity and reliability of your vehicle's electrical and engine management systems.

Frequently Asked Questions About P0685 Code

• Can I drive my car with a P0685 code?
  It's not recommended to drive with a P0685 code. The issue can cause your engine to stall or fail to start, potentially leaving you stranded.
• How urgent is it to fix a P0685 code?
  Very urgent. This code indicates a problem with the PCM's power supply, which is critical for proper engine operation. Address it as soon as possible to prevent further damage.
• Will a P0685 code clear itself?
  No, the P0685 code will not clear itself. The underlying issue must be fixed, and then the code needs to be cleared using an OBD-II scanner.
• How much does it typically cost to fix a P0685 code?
  Costs can vary widely, from $50 for a simple relay replacement to over $1000 for PCM issues. The average repair cost is typically between $200 and $500.
• Can a bad battery cause a P0685 code?
  While a bad battery itself doesn't directly cause P0685, it can contribute to voltage issues that may trigger the code. Always check the battery condition during diagnosis.
• Is it safe to clear the P0685 code without fixing the problem?
  No, it's not safe. Clearing the code without addressing the underlying issue can lead to repeated occurrences and potential engine damage.
• Can weather conditions affect the occurrence of P0685 codes?
  Yes, extreme temperatures and moisture can exacerbate electrical issues. Cold weather, in particular, can strain the battery and electrical system, potentially triggering P0685.
• How long does it take to diagnose and fix a P0685 code?
  Diagnosis typically takes 1-2 hours. Repair time varies based on the issue, ranging from 30 minutes for a simple relay replacement to several hours for complex wiring or PCM problems.

In conclusion, the P0685 code is a serious issue that requires prompt attention. While some aspects of diagnosis and repair can be performed by experienced DIY mechanics, complex electrical issues or PCM-related problems are best left to professionals. Always prioritize safety and use proper diagnostic procedures when dealing with any engine code. By understanding the causes, symptoms, and solutions for the P0685 code, you can make informed decisions about your vehicle's maintenance and repairs, ensuring its longevity and reliability.