As an automotive content creator for autelfrance.com and a seasoned auto repair expert, I understand the frustration of encountering OBD-II error codes. Among these, the P0685 code, often described as “ECM/PCM Power Relay Control Circuit Open,” is a common issue that can leave you stranded. This article provides a comprehensive guide to understanding, diagnosing, and resolving the P0685 error code, ensuring you’re well-informed to tackle this problem effectively.
Decoding the P0685 Error Code: ECM/PCM Power Relay Explained
The P0685 diagnostic trouble code is a generic powertrain code, meaning it is applicable across a wide range of vehicles manufactured from 1996 onwards, including brands like Honda, Ford, VW, GM, Chrysler, and more. While the code definition remains consistent, the specific causes can vary slightly depending on the vehicle’s make and model.
In essence, the P0685 code indicates that the vehicle’s Powertrain Control Module (PCM) or Engine Control Module (ECM) is not receiving power as expected. Modern vehicles rely on relays to supply power to the PCM/ECM, which is the brain of your car’s engine management system. This relay, known as the PCM power relay, is crucial for providing the necessary voltage for the PCM to operate correctly.
Alt: Diagram illustrating OBD-II Trouble Code P0685 ECM/PCM Power Relay Control Circuit Open in a vehicle’s electrical system.
The PCM power relay typically has five terminals:
- Primary Input Terminal: Receives a constant supply of battery voltage.
- Ground Terminal: Connected to a ground point on the engine or chassis.
- Secondary Input Terminal: Supplied with battery voltage via a fused circuit when the ignition switch is turned “ON”.
- Output Terminal: Provides voltage to the PCM.
- Signal Wire (CAN): For communication with the Controller Area Network.
When you turn the ignition switch to the “ON” position, the secondary input terminal energizes a small coil inside the relay. This energization closes the relay contacts, completing the circuit and allowing battery voltage to flow from the primary input terminal, through the relay, and out to the output terminal, ultimately powering the PCM. The P0685 code is triggered when the PCM detects an “open” circuit, meaning a lack of voltage or insufficient voltage in this critical power supply circuit.
Recognizing the Symptoms of a P0685 Error
The most prominent symptom associated with a P0685 error code is a no-start condition. If your engine fails to crank or start when this code is present, it’s a strong indicator of a P0685 issue. In such cases, addressing this code becomes a necessity, not just an option.
However, in rarer instances, the engine might still start and run even with a P0685 code stored. If this occurs, it could point towards a less severe intermittent fault in the power relay circuit, or potentially indicate a malfunction within the PCM itself, or even a PCM programming error. It’s crucial to not ignore the code even if the car seems to be running, as underlying issues can worsen over time.
Common Causes Behind the P0685 Code
Several factors can contribute to the triggering of a P0685 error code. Identifying the root cause is essential for effective repair. Here are the potential culprits:
- Defective PCM Power Relay: The relay itself might be faulty, failing to close the circuit and provide power to the PCM. This is a very common cause.
- Blown Fuse or Fusible Link: A blown fuse in the PCM power relay circuit or a damaged fusible link can interrupt the voltage supply.
- Corroded or Damaged Wiring/Connectors: Wiring issues, particularly near the PCM relay, can cause open circuits or voltage drops. Corrosion, fraying, or damage to wires and connectors should be inspected.
- Faulty Ignition Switch: A malfunctioning ignition switch, specifically the electrical portion, might not properly send power to the PCM relay when turned to the “ON” position.
- Disconnected Electrical Terminal at Ignition Switch: A loose or completely disconnected terminal at the ignition switch can break the circuit.
- Loose or Corroded Battery Cable Ends: While less direct, poor battery connections can sometimes contribute to voltage irregularities and trigger seemingly unrelated codes.
Alt: Image depicting corroded automotive wiring and connectors, a potential cause for P0685 error code due to circuit interruption.
Step-by-Step Diagnostic and Repair Procedures for P0685
Diagnosing a P0685 code requires a systematic approach. Here’s a breakdown of the diagnostic and repair process:
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Check for Technical Service Bulletins (TSBs): Begin by checking for any TSBs related to your vehicle and the P0685 code. Manufacturers often release TSBs for known issues and their fixes, which can save you considerable time.
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Visual Inspection: Conduct a thorough visual inspection of all accessible wiring harnesses, connectors, and components, particularly around the PCM power relay. Look for:
- Relays that are loose or improperly seated.
- Corrosion on relay blades or terminals.
- Damaged or frayed wiring.
- Loose or corroded battery terminals.
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Gather Necessary Tools: You’ll need the following tools for diagnosis:
- OBD-II Scanner (or Code Reader): To read and clear codes.
- Digital Volt/Ohmmeter (DVOM): To test voltage and circuit continuity.
- Wiring Diagram: Essential for understanding the PCM power relay circuit layout in your specific vehicle. Wiring diagrams can be found in service manuals or online databases like All Data.
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Retrieve and Record Trouble Codes: Use your scanner to retrieve all stored trouble codes and freeze frame data. Record these for future reference, especially if the issue is intermittent. Clear the P0685 code and attempt to start the vehicle to see if it returns immediately.
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Voltage Testing at the PCM Power Relay:
- Primary Input Terminal: Using your DVOM, verify battery voltage at the primary input terminal of the PCM power relay. Consult your wiring diagram to identify the correct terminal. If no voltage is present, check fuses and fusible links upstream in the circuit.
- Secondary Input Terminal: Test for voltage at the secondary input terminal with the ignition switch in the “ON” position. No voltage here suggests a blown fuse or a faulty ignition switch (electrical portion).
- Ground Signal: Check for a proper ground signal at the relay’s ground terminal. A lack of ground can indicate issues with system ground points, chassis grounds, or battery cable connections.
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Output Voltage Test: If the input and ground signals are correct, test for output voltage at the relay’s output terminal when the ignition is “ON”. No output voltage with correct inputs points to a faulty relay.
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Voltage at PCM Connector: If the relay output is good, test for voltage at the PCM connector itself, at the pin that receives power from the relay. No voltage here indicates a wiring problem between the relay and the PCM. Inspect the wiring for breaks, shorts, or damage. Disconnect controllers before testing resistance to avoid damaging them.
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PCM Suspect (Last Resort): If voltage is reaching the PCM connector, and all other checks are normal, the PCM itself may be defective or require reprogramming. This is less common and should be considered after ruling out all other possibilities.
Important Notes During Diagnosis:
- Ignition Switch (Electrical Portion): When referring to the ignition switch, focus on the electrical component, not the mechanical key tumbler.
- Relay Swapping: If possible, temporarily swap the PCM power relay with an identical relay from another non-critical system (like the horn relay, if specifications match) to test relay functionality. Always return relays to their original positions and replace the faulty relay with a new one.
- Fuse Testing: Always test fuses under load (with the circuit energized as much as safely possible) for the most accurate results.
Seeking Further Assistance
While this guide provides a comprehensive approach to diagnosing and resolving the P0685 error code, complex electrical issues can sometimes be challenging. If you require further assistance, consult a qualified automotive technician. You can also find valuable community support and discussions in online car repair forums dedicated to OBD-II codes and vehicle diagnostics.
Disclaimer: This information is for informational purposes only and should not be considered professional repair advice. Always prioritize safety and consult with qualified professionals when working on vehicle electrical systems. We are not responsible for any actions taken based on this information.
