P0685: ECU Power Relay Control Circuit Open

Introduction

The DTC P0685 trouble code indicates a problem with the engine control unit (ECU) power relay control circuit. This code is common in modern vehicles and can affect the operation of the engine, causing a variety of symptoms that can confuse even the most experienced mechanics. In this article, we will explore in detail the symptoms, causes, diagnosis, and possible solutions to resolve the problem associated with this trouble code.

Symptoms

When a P0685 error occurs, the driver may notice several symptoms, including:

  • Check Engine Light (MIL) on dashboard comes on
  • Difficulty starting the engine
  • Engine suddenly stalls
  • Erratic engine behavior while driving
  • Failure of diagnostic tools to communicate with the engine control unit

These symptoms can vary in severity and may be accompanied by other trouble codes, such as P0686 (ECU Power Relay Control Circuit Low) and P0687 (ECU Power Relay Control Circuit High).

Cause

The P0685 code can be caused by a number of problems, including:

  • Faulty power relay :
    If the relay that powers the engine control unit is faulty, the ECU may not receive the power it needs to function properly.

  • Blown fuse :
    A blown fuse in the power circuit can interrupt power to the engine control unit.

  • Damaged cables or connectors :
    Damaged wires, loose or corroded connectors can prevent the circuit from functioning properly.

  • Problems in the engine control unit :
    In rare cases, an internal ECU malfunction can cause this error.

  • Land problems :
    A poor ground connection can affect the control unit's ability to receive the necessary power.

Diagnosis

Diagnosing the P0685 code requires a systematic approach. Here are the steps to follow:

  1. Scanning for error codes :
    Use an OBD-II diagnostic tool to read stored error codes. Note any related codes.

  2. Visual inspection :
    Inspect the wiring and connectors in the ECU power circuit. Make sure there are no signs of wear, corrosion or damage.

  3. Checking the fuse :
    Check the fuse associated with the engine control unit power relay. Replace if necessary.

  4. Relay Test :
    Use a multimeter to test the power relay. Verify that it is working properly and that it switches power when activated.

  5. Power Control :
    Measure the voltage entering and exiting the power relay to ensure the ECU is receiving adequate power.

  6. Control unit test :
    If everything else is in order, you may need to test the engine control unit to make sure there are no internal problems.

Resolution and Repair

Once you have identified the cause of the P0685 code, you can proceed with the repair. Here are some of the common solutions:

  • Replacing the power relay :
    If the power relay is defective, it must be replaced with a new one.

  • Wiring repair or replacement :
    If cables or connectors are damaged, repair or replace them to ensure a proper electrical connection.

  • Replacing the fuse :
    If the fuse is blown, replace it with a new one of the same amperage.

  • Earthing check :
    Make sure all ground points are clean and well connected to ensure proper operation of the control unit.

  • Replacing the engine control unit :
    If the engine control unit is faulty, it may need to be replaced or repaired.

Insight into the Electrical Components Involved

Power Relay and Circuit

The power relay plays a crucial role in the operation of the engine control unit. It acts as an electronic switch that connects the power supply to the ECU, and without a proper functioning of the relay, the ECU cannot operate.

  • Relay function :
    The relay is activated by a signal from the control unit itself or another sensor. When the relay is activated, it allows the passage of electrical current to the control unit.

  • Common problems :
    Relays can fail due to overheating, wear, or contact problems. It is important to check that they are working properly, especially in older vehicles.

  • Wiring :
    The circuit connecting the relay to the control unit must be carefully examined. A loose or corroded wire can also cause power problems.

Conclusions

The DTC P0685 trouble code is an indicator of a problem in the engine control unit power relay control circuit. Diagnosing and fixing this problem requires attention to detail and a systematic approach. Monitoring symptoms, performing a thorough diagnosis, and resolving identified issues is critical to restoring your vehicle to proper operation. Always remember to perform thorough visual checks and do not underestimate the importance of the electrical components involved. With proper experience and the right tools, you can resolve the problem and ensure the safety and reliability of your vehicle.

Important
Not all OBD2 codes used by one manufacturer are used by other brands, and may not have the same meaning.
The information presented here is for informational purposes only. We are not responsible for any actions you take on your vehicle. If you have any concerns about repairing your vehicle, please contact your repair shop.