Decoding the OBD-II P0685 Code: ECM/PCM Power Relay Control Circuit Open

The OBD-II P0685 code, often accompanied by its European counterpart EOBD P0685, signals a critical issue within your vehicle’s engine management system. As an expert at Autel France, specializing in automotive diagnostics, I frequently encounter this trouble code, which points to a problem in the ECM/PCM power relay control circuit. This article provides an in-depth look at this diagnostic trouble code, ensuring you understand its implications, symptoms, causes, and effective diagnostic and repair procedures.

Understanding the P0685 Code: ECM/PCM Power Relay Control Circuit Open

The P0685 code is a generic powertrain code, meaning it can occur in 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 across manufacturers, the specific reasons behind it can vary slightly depending on the vehicle’s make and model.

In essence, when your vehicle’s Powertrain Control Module (PCM) stores the P0685 code, it indicates that the module has detected an open circuit or a low/no-voltage condition in the control circuit for the PCM power relay. This relay is crucial as it’s responsible for supplying battery voltage to the PCM, the brain of your engine management system. Some vehicles might utilize a fused circuit instead of a relay, but relays are more common, typically featuring a five-terminal design.

Let’s break down how this relay typically functions:

• 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 (often through a fuse) when the ignition switch is turned to the “ON” position.
• Output Terminal: Provides voltage to the PCM.
• Signal Wire (CAN): May be present for communication within the Controller Area Network.

When you turn your ignition to “ON,” a small coil within the relay energizes. This energization closes the relay’s internal contacts, completing the circuit and allowing battery voltage to flow to the PCM via the output terminal. The P0685 code is triggered when this voltage supply is disrupted.

Recognizing the Symptoms of a P0685 Fault

The most prominent symptom associated with a P0685 code is a no-start condition. If your engine cranks but fails to start, and a P0685 code is present, it’s highly likely that the fault is directly related to this code. In such scenarios, addressing the P0685 code becomes imperative to get your vehicle running again.

However, in less common instances, you might encounter a situation where the P0685 code is stored, yet the engine starts and runs normally. In these rarer cases, suspect a potential faulty PCM or a PCM programming error. It’s always crucial to thoroughly investigate the code, even if the most obvious symptom – the no-start condition – is absent.

Pinpointing the Potential Causes of P0685

Several factors can lead to the setting of a P0685 trouble code. Here’s a breakdown of the common culprits:

• Defective PCM Power Relay: The relay itself might be faulty, failing to energize or close its contacts properly.
• Blown Fuse or Fusible Link: A blown fuse in the PCM power supply circuit or a damaged fusible link can interrupt voltage flow.
• Corroded or Damaged Wiring or Connectors: Wiring issues, particularly near the PCM relay and connectors, can cause open circuits or voltage drops. Pay close attention to areas exposed to moisture or vibration.
• Faulty Ignition Switch (Electrical Portion): A malfunctioning ignition switch might fail to properly send voltage to the PCM relay when turned to the “ON” position.
• Partially or Totally Disconnected Electrical Terminal at the Ignition Switch: Loose or disconnected terminals at the ignition switch can disrupt the power supply.
• Loose or Corroded Battery Cable Ends: Poor battery connections can impact overall system voltage and contribute to relay malfunction.

Step-by-Step Diagnostic and Repair Procedures for P0685

When diagnosing a P0685 code, a systematic approach is crucial. Here’s a detailed procedure to guide you:

1. Preliminary Checks & TSBs: Always begin with a check for Technical Service Bulletins (TSBs) specific to your vehicle’s make and model. Manufacturers often issue TSBs to address known issues, and a TSB related to P0685 could save you significant diagnostic time.

2. Visual Inspection: Conduct a thorough visual inspection of all wiring harnesses, connectors, and components related to the PCM power supply circuit. Pay particular attention to:

   • Relays: Ensure the PCM power relay is securely seated in its socket. Look for signs of corrosion on the relay blades or within the socket terminals. Relays located near the battery or coolant reservoir are particularly susceptible to corrosion.
   • Wiring: Inspect for damaged, chafed, or corroded wiring, especially around connectors and near the PCM relay.
   • Fuses & Fusible Links: Check all relevant fuses and fusible links in the PCM power supply circuit for any signs of being blown or damaged.
   • Battery Terminals: Examine the battery terminals for tightness and excessive corrosion. Clean and tighten as necessary.

   Repair or replace any identified defects during your visual inspection.

3. Gather Necessary Tools & Information: You’ll need the following tools and resources:

   • OBD-II Scanner (or Code Reader): To retrieve diagnostic trouble codes and freeze frame data.
   • Digital Volt/Ohmmeter (DVOM): For electrical circuit testing.
   • Wiring Diagram: Obtain a wiring diagram specific to your vehicle, detailing the PCM power supply circuit. These diagrams are usually available in the vehicle’s service manual or through online automotive repair information services like AllData. Ensure the diagram includes the PCM power supply circuit.

4. Retrieve Diagnostic Codes & Freeze Frame Data: Connect your scanner to the vehicle’s diagnostic port and retrieve all stored trouble codes. Record these codes for future reference. Note any freeze frame data associated with the P0685 code, as this information can be valuable if the fault is intermittent. Clear the codes and attempt to restart the vehicle to see if the P0685 code returns.

5. Power Relay Circuit Testing: Focus on testing the PCM power relay circuit. Refer to your wiring diagram and the relay terminal identification (often found on the relay itself or in the service manual).

   • Primary Input Terminal Voltage: Using your DVOM, check for battery voltage at the primary input terminal of the PCM power relay. If no voltage is present, investigate upstream fuses or fusible links in the power supply path.
   • Secondary Input Terminal Voltage: Test for voltage at the secondary input terminal with the ignition switch in the “ON” position. If no voltage is present, suspect a blown fuse or a faulty ignition switch (electrical portion).
   • Ground Signal: Verify a good ground signal at the relay’s ground terminal. If no ground is present, check system ground locations, bulkhead connectors, chassis grounds, and battery cable connections.
   • Output Voltage: If the input and ground circuits are confirmed to be good, test for output voltage at the relay’s output terminal when the ignition is “ON”. No output voltage, despite good inputs, strongly suggests a faulty relay.

6. PCM Connector Voltage Test: If the relay output is confirmed, test for system voltage at the PCM connector itself, specifically at the pins designated for power supply. If voltage is absent at the PCM connector, suspect a wiring issue between the relay output and the PCM. Carefully inspect the wiring harness for breaks, shorts, or corrosion. Disconnect system controllers from the harness before performing resistance tests with your DVOM to avoid damage. Repair or replace any identified open or shorted circuits.

7. PCM as Last Resort: If voltage is present at the PCM power supply pins, and all wiring and relay tests are satisfactory, then a defective PCM or a PCM programming error becomes a likely possibility. PCM replacement or reprogramming might be necessary, which often requires specialized tools and expertise.

Important Notes During Diagnosis:

• Ignition Switch (Electrical Portion): When referring to the “ignition switch,” focus on the electrical portion of the switch responsible for sending signals and power, not the mechanical key cylinder.
• Relay Swapping: If you suspect a faulty relay, swapping with an identical relay (same part number and function) can be a helpful diagnostic step. However, always return relays to their original positions and replace the suspected faulty relay with a new one after testing.
• Fuse Testing Under Load: When testing fuses, ensure the circuit is under a load or at maximum voltage to accurately assess fuse integrity. A fuse that appears good visually might still fail under load.

By following these detailed diagnostic steps, you can systematically troubleshoot a P0685 code and effectively pinpoint the root cause, leading to a successful repair. Remember, while this guide provides comprehensive information, always consult your vehicle-specific service manual for detailed wiring diagrams and component locations. If you are uncomfortable performing these diagnostic procedures yourself, it is always best to seek assistance from a qualified automotive technician.

For further discussions and support regarding the P0685 code, feel free to visit our car repair forums where you can post questions and interact with other automotive professionals and enthusiasts.

Disclaimer: This information is for informational purposes only and should not be considered as professional repair advice. We are not responsible for any actions taken based on this information. Always prioritize safety and consult with qualified professionals when performing vehicle repairs.