Introduction

The DTC P0686 trouble code is associated with a problem in the Engine Control Unit (ECU) power relay control circuit. The Engine Control Unit is the electronic brain of a vehicle, and the power relay provides the power it needs to function properly. When the power relay circuit has a low signal or fails to function properly, the ECU does not receive the power it needs, resulting in engine failure.

DTC Code P0686 Description

Code P0686: Engine Control Unit Power Relay Circuit Low

• System involved: Engine management system, ECU power relay
• Common Symptoms: Engine Light (MIL) On, Engine Won't Start, Rough Starting, Engine Stalling Suddenly
• Main cause: Engine control unit power relay failure, wiring problems or faulty fuses

Engine Control Unit Power Relay Operation

The power relay is a critical electrical component located between the vehicle's battery and the engine control unit. The relay's primary function is to allow power to flow from the battery to the ECU when the vehicle is running. The engine control unit regulates engine management, including ignition, fuel injection, and other key functions. If the relay does not function properly, the ECU will not receive the power it needs, leading to various engine running problems or even the inability to start the vehicle.

Relay circuit operation

When you turn on your car, the ECU sends a signal to the power relay, which activates the circuit and allows power to flow from the battery to the ECU. If this signal is insufficient (too low or missing), the ECU cannot control the engine properly, causing the P0686 code.

Causes of Code P0686

DTC code P0686 is stored when the engine control unit power relay circuit signal is lower than the manufacturer's default values. This can be due to several reasons, including:

1. Faulty power relay: The relay may be damaged or worn, preventing proper power flow to the ECU.
2. Faulty Wiring: The wires connecting the battery, relay, and ECU may be worn, corroded, or disconnected, causing a broken circuit.
3. Blown Fuses: A faulty fuse in the relay circuit can prevent current from flowing.
4. Ground Problems: A poor ground connection can affect the relay's ability to function properly.
5. Faulty Engine Control Unit: In some cases, an internal ECU fault can cause the P0686 code to activate, although this is less common.

Symptoms of Code P0686

When the P0686 error code occurs, you may observe a number of symptoms that may vary depending on the severity of the problem. Here are the most common symptoms associated with this code:

• Engine does not start: If the power relay is not working properly, the engine control unit will not receive power and the engine will not start.
• Hard or Rough Starting: The vehicle may have difficulty starting or may start intermittently.
• Sudden Engine Stop: The engine may suddenly stop while driving, due to loss of power to the control unit.
• Engine Warning Light (MIL) On: The engine warning light will illuminate on the dashboard, signaling a problem.
• Electrical Operation Problems: Malfunctions or irregularities in other electronic functions of the vehicle may occur because the ECU is not receiving stable voltage.

Diagnostic Procedure

To resolve the P0686 code, it is necessary to perform a thorough diagnosis that will identify the cause of the power relay circuit malfunction. The following diagnostic procedure is recommended:

1. Reading the error code

Use an OBD-II scanner to confirm the presence of the P0686 code and check for other related codes that may help identify the problem. Check for any codes that relate to the electrical system or ECU.

2. Visual inspection of power relay and fuses

Check the power relay and make sure it is not visibly damaged or worn. Verify that the relay circuit fuse is intact and not blown. If the fuse is blown, replace it and see if the problem persists.

3. Power relay test

Use a multimeter to check the operation of the power relay. Verify that the relay closes properly when activated and that it allows current to flow to the ECU.

4. Check wiring and connections

Carefully inspect the wiring and connectors connecting the battery, relay and ECU. Look for signs of corrosion, wear or loose connections. Verify that all connections are secure and well insulated.

5. ECU signal check

If the relay and wiring are OK, you may need to check if the ECU is sending the correct signal to the power relay. This requires an advanced diagnostic tool that can read and monitor the electrical signals from the ECU.

6. Check the supply voltage

Check the voltage supplied from the battery to the relay and from the relay output to the engine control unit. Insufficient voltage may indicate a problem in the fuel system, while irregular voltage may be caused by a ground problem.

Solutions and Repairs

After completing the diagnosis, the following solutions may resolve the P0686 code:

1. Replacing the power relay

If the power relay is faulty or malfunctioning, the simplest solution is to replace it. A faulty relay will not provide the necessary voltage to the engine control unit and will prevent the engine from operating properly.

2. Repair or replacement of wiring

If the wiring or connectors show signs of wear or damage, they should be repaired or replaced to ensure proper current flow between the battery, relay and ECU.

3. Replacing blown fuses

If one of the power circuit fuses is blown, replace it with one of the correct rating. A bad fuse can prevent the power relay from working properly.

4. Mass check and repair

Make sure the vehicle's ground system is in good condition. A problem with the ground can cause an interruption in the flow of power and lead to the activation of the P0686 code.

5. Repair or replacement of the engine control unit

In rare cases, the problem may be caused by a malfunction of the engine control unit. If after all checks the P0686 code persists, the ECU may need to be repaired or replaced.

Prevention

To prevent the recurrence of the P0686 code, some preventive measures can be adopted:

1. Regular electrical system maintenance: Periodically checking the condition of wiring, fuses and power relays can prevent future problems.

2. Check Battery and Ground: A weak battery or faulty ground system can cause power problems. Make sure that

   the battery is in good condition and that all ground connections are secure.

3. Inspection of electrical connections: Regularly check the condition of electrical connections to prevent corrosion or deterioration of contacts.

Conclusions

DTC P0686 is an indicator of a problem with the engine control unit power relay. This fault can cause starting problems, sudden shutdowns, and other engine management system issues. Fixing the code requires a thorough diagnosis of the relay, wiring, and associated fuses. With proper maintenance, you can prevent future problems and keep your engine running smoothly.