Understanding the P0685 Trouble Code
The P0685 Code is a common OBD-II (On-Board Diagnostics II) trouble code that is classified as a generic powertrain code. This means it can occur in a wide range of vehicles manufactured from 1996 onwards, across various makes and models, including but not limited to Honda, VW, Ford, Dodge, Chrysler, Acura, Audi, and GM. While the code is generic in nature, the specific causes and diagnostic steps can vary slightly depending on the vehicle’s make and engine configuration.
In many instances, the presence of a P0685 code is directly linked to a no-start condition. When your vehicle’s Powertrain Control Module (PCM) stores this code, it indicates that the system has detected an issue within the circuit responsible for supplying battery voltage to the PCM. Specifically, it signals a low voltage or an open circuit in the ECM/PCM power relay control circuit.
Modern vehicles often employ a relay to manage the power supply to the PCM. However, some older or simpler systems might utilize a fused circuit instead. A typical PCM power relay is a five-terminal component. It receives a constant battery voltage input, connects to a ground (often engine or chassis ground), and has a secondary input terminal that gets battery voltage (through a fuse) when the ignition is turned to the “ON” position. The relay also features an output terminal that delivers power to the PCM and, in some cases, a signal wire for the Controller Area Network (CAN).
When you turn the ignition key to the “ON” position, a small coil inside the PCM power relay is energized. This energization causes the relay’s internal contacts to close, completing the electrical circuit. This action allows battery voltage to flow to the output terminal and subsequently power the PCM, enabling the engine management system to function.
Image: Diagram illustrating a typical PCM power relay and its connections.
Symptoms Associated with a P0685 Code
The most prominent symptom associated with a P0685 code is usually a no-start condition. If your vehicle is experiencing a P0685 code and fails to start, it is crucial to address this issue promptly. Ignoring this code is generally not a viable option as it directly impacts the vehicle’s ability to run.
In rarer scenarios, you might encounter a P0685 code even when the engine starts and runs normally. In such cases, consider the possibility of a faulty PCM itself or a PCM programming error. However, these instances are less frequent compared to the no-start situation.
Common Causes of a P0685 Code
Several potential issues can trigger a P0685 code. Here’s a breakdown of the common causes:
- Defective PCM Power Relay: The relay itself might be faulty, failing to properly close the circuit and supply power to the PCM. This is a very common cause for the P0685 code.
- Blown Fuse or Fusible Link: A blown fuse or a damaged fusible link in the PCM power supply circuit will interrupt the voltage flow, preventing the relay from energizing or power from reaching the PCM.
- Corroded or Damaged Wiring or Connectors: Wiring issues, including corrosion, fraying, or damage, particularly in the vicinity of the PCM relay and its related circuits, can lead to an open circuit. Pay close attention to connectors, ensuring they are clean and securely connected.
- Faulty Ignition Switch: The ignition switch, specifically its electrical portion, plays a critical role in activating the PCM power relay. A faulty ignition switch may fail to send the signal required to energize the relay.
- Partially or Totally Disconnected Electrical Terminal at the Ignition Switch: Loose or disconnected terminals at the ignition switch can disrupt the signal flow to the PCM power relay, leading to a P0685 code.
- Loose or Corroded Battery Cable Ends: Poor battery connections, whether due to looseness or corrosion, can affect the overall electrical system and contribute to PCM power supply issues, indirectly causing a P0685 code.
Image: Example of a corroded battery terminal, a potential cause of electrical issues.
Diagnostic and Repair Steps for a P0685 Code
When diagnosing a P0685 code, a systematic approach is essential. Here’s a step-by-step guide to help you troubleshoot and repair the issue:
1. Preliminary Checks and Technical Service Bulletins (TSBs):
- Begin your diagnosis by checking for Technical Service Bulletins (TSBs) specific to your vehicle’s make, model, and year. Manufacturers often issue TSBs to address known issues, and there might be a TSB related to the P0685 code or PCM power supply problems for your vehicle. Checking TSBs can save you valuable time by pointing you towards a known fix.
2. Visual Inspection:
- Conduct a thorough visual inspection of the wiring harnesses, connectors, and components related to the PCM power supply system.
- Pay close attention to the PCM power relay itself. Check if it is securely seated in its socket. Look for signs of corrosion on the relay blades or terminals, especially if the relay or fuse box is located near the battery or coolant reservoir, where moisture and corrosive substances are more likely to accumulate.
- Inspect the battery terminals and cable ends for tightness and corrosion. Clean and tighten them if necessary.
- Examine the fuses associated with the PCM power circuit. Check for blown fuses. Refer to your vehicle’s owner’s manual or repair manual for fuse locations.
3. Gather Diagnostic Tools and Information:
- You will need a few essential tools for diagnosing a P0685 code:
- OBD-II Scanner or Code Reader: To confirm the presence of the P0685 code and retrieve any other stored trouble codes.
- Digital Volt/Ohmmeter (DVOM): To perform voltage and continuity tests on electrical circuits.
- Wiring Diagram: A wiring diagram for your vehicle’s PCM power supply circuit is crucial. You can find these in a manufacturer’s service manual or through online subscription services like All Data. Ensure the diagram covers the PCM power supply circuit.
4. Initial Scan and Data Recording:
- Use your OBD-II scanner to retrieve all stored trouble codes. Record these codes for future reference.
- Note any freeze frame data associated with the P0685 code. Freeze frame data captures the engine parameters at the moment the code was set, which can provide valuable clues about the conditions that triggered the fault.
5. PCM Power Relay Circuit Testing:
- Locate the PCM Power Relay: Refer to your vehicle’s service manual or wiring diagram to pinpoint the exact location of the PCM power relay.
- Test Primary Input Terminal Voltage: Using your DVOM, check for battery voltage at the primary input terminal of the PCM power relay. Consult your wiring diagram or a pin-out chart in the service manual to identify the correct terminal. If there’s no voltage, suspect a problem with the fuse or fusible link supplying power to this terminal.
- Test Secondary Input Terminal Voltage: Test for voltage at the secondary input terminal of the relay with the ignition switch in the “ON” position. If no voltage is present, suspect a blown fuse in this circuit or a fault within the ignition switch (electrical portion).
- Check Ground Signal: Verify the ground signal at the relay’s ground terminal. If there is no ground, inspect system ground locations, harness connectors, chassis grounds, and battery cable connections.
- Test Output Voltage: If the input and ground circuits are normal, test for output voltage at the relay’s output terminal when the ignition is “ON”. If there is no output voltage, and you’ve confirmed power and ground to the relay, suspect a faulty PCM power relay itself. You can try swapping it with an identical relay from another less critical system (like the horn relay, if it’s the same type) to see if the problem resolves. Always return relays to their original positions and replace the suspected faulty relay with a new one.
6. PCM Connector Voltage Test:
- If the relay output is confirmed, test for system voltage at the PCM connector itself. If there’s no voltage at the PCM connector, there’s likely an open circuit in the wiring between the relay and the PCM.
7. Wiring Circuit Testing:
- If voltage is missing at the PCM connector, you need to test the wiring harness for continuity and shorts. Always disconnect system controllers (like the PCM) from the harness before performing resistance tests with a DVOM to prevent damage to the controllers.
- Check for open circuits (breaks in the wire) and short circuits (wires shorted to ground or to each other) in the wiring between the PCM power relay and the PCM. Repair or replace any damaged wiring as needed.
8. PCM as a Last Resort:
- If you have confirmed that power and ground are reaching the PCM, and all wiring is intact, and the relay is functioning correctly, then consider the possibility of a defective PCM or a PCM programming error. PCM failures are less common than relay or wiring issues, but they can occur. PCM replacement or reprogramming should be considered as a last resort after thoroughly ruling out all other possibilities.
Important Notes:
- When referring to the “ignition switch,” we are specifically talking about the electrical portion of the switch, not the mechanical key tumbler.
- Swapping identical relays for testing purposes can be a helpful diagnostic step, but always ensure you are using relays with matching part numbers and return them to their original positions after testing.
- When testing fuses, make sure the circuit is under maximum voltage conditions, meaning the ignition should be in the “ON” position for circuits that are activated by the ignition.
Image: A technician using a multimeter to test electrical circuits, a crucial step in diagnosing electrical issues like P0685.
Need More Assistance with a P0685 Code?
If you require further assistance in diagnosing and resolving a P0685 trouble code, we encourage you to post your question in our free car repair forums. Our community of experienced mechanics and automotive enthusiasts is ready to offer guidance and support.
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Disclaimer: This information is provided for informational purposes only and should not be considered as professional repair advice. We are not responsible for any actions you take on your vehicle based on this information. Automotive repair can be complex and should be performed by qualified technicians.
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