P0686 Diagnostic Trouble Code: ECM/PCM Power Relay Control Circuit Low

Fault code P0686 is called “ECM/PCM Power Relay Control Circuit Low” but in different programs it may be called differently. This fault designation applies to all vehicles equipped with OBD-II.

Technical description and explained code P0686

If your OBD-II equipped vehicle has stored code P0686, it means that the control unit has detected a low signal level in the circuit going to the ECM/PCM main power relay. That is, the transmission control module has detected that its own power supply has a low signal level problem.

P0686 Diagnostic Trouble Code: ECM/PCM Power Relay Control Circuit Low

Most PCM power circuits are relatively simple and straightforward. When the ignition is turned to the “ON” position, power is applied to the relay. This causes the solenoid to close two points of contact, one of which is permanently connected to the battery power source. When this occurs, the circuit closes and power from the battery is applied to the PCM. Additionally passing through the circuit with a fuse or fusible link.

Relays usually have a five pin design. The primary input pin is supplied with DC battery voltage. The ground output, grounded to the engine or chassis ground.

The secondary input terminal is supplied with battery voltage through a fused circuit when the ignition switch is in the “ON” position. The fourth terminal is the output for the PCM. The fifth, is the signal wire for the Controller Area Network (CAN).

The electronic control systems in vehicles equipped with OBD-II are controlled by a network of computers called control modules. This involves constant communication between the various modules via the Controller Area Network (CAN).

Each time the engine is started, the PCM performs a self-test of all the controllers. If the normal voltage input signal is lower than the reference signal. Code P0686 will be stored and the malfunction indicator light may illuminate.

Symptoms of vehicle malfunctions

The main signal that an error P0686 has occurred is the Malfunction Indicator Lamp (MIL) is also known as the CheckEngine Light.

It can also be warning signs such as:

  1. The “Check engine” control lamp on the control panel will light up (the code will be stored as a fault).
  2. Other related fault codes may also be present.
  3. Transmission emergency mode to prevent further damage.
  4. There may be problems starting the engine.
  5. There may be a drop in engine power.
  6. In most cases the engine will not start at all.

The diagnosis code P0686 is considered serious, as the engine cannot be started in most cases. If this code is present, but the engine starts and runs. Most likely there is a problem with the PCM programming or the module itself.

Factors that can cause this error code

The error code P0686 can mean that one or more of the following problems have occurred:

  • Faulty PCM power relay.
  • Fuse blown.
  • Open or short circuit in CAN circuit.
  • Damaged, corroded wiring or connectors.
  • Low system voltage.
  • Partially or completely disconnected electrical terminal on ignition switch.
  • Defective ignition switch.
  • Faulty PCM or other control module, also possible programming error.

How to fix or reset OBD-2 code P0686

Some suggested steps for troubleshooting and fix the error code P0686:

  1. Inspect the wiring for damage, abrasions, breaks, and shorts. If wiring is damaged, repair or replace it.
  2. Remove all diagnostic trouble codes from the PCM and perform a test drive to see if diagnostic trouble code P0686 returns.
  3. If the code returns, check the ECM/PCM relay to make sure it is working properly.
  4. Inspect and check the fuse and fusible link.
  5. Check the wiring diagram of the relay and fuse system.
  6. Clear the fault codes again and perform a test drive.
  7. Insert the key into the ignition switch and turn it to the on position. If the Check Engine light does not come on, but the code in memory appears again, the relay may be faulty.
  8. If the relay is faulty, it may be necessary to replace and reprogram the PCM.

Diagnose and repair of problems

To exclude all variants of error P0686, you should visually inspect the wiring, check the voltage on the battery. Also check the battery terminals and cables for loose connections.

Ensure that the system voltage is as specified in the manual. The charging system is fully functional and the battery is in good condition, begin the diagnostic procedure.

Locate the starting point of the ECM/PCM supply circuit. This is usually the ignition switch, so use a digital multimeter to check the voltage at this wire or terminal. Compare the reading with the value given in the manual, and repair if necessary if an abnormality is found.

If the code remains, but the power supply to and from the ignition switch is within the specified range. Note and test the power supply to the fuse box and then to the PCM connector. If abnormalities are found, make the necessary repairs. To ensure that all electrical parameters are in accordance with the manufacturer’s specifications.

The steps described above will help clear error P0686, but in some cases where the cause is not obvious, intermittent faults may be present. Also, if the PCM is already damaged you may need to have it repaired by a specialized repair shop.

On which vehicles does this problem occur most frequently

Fault code P0686 can occur on different vehicles but there are statistics on which brands this occurs most often. Here is a list of some of them:

  • Acura
  • Audi
  • BMW
  • Chevrolet (Cruze, Spark)
  • Chrysler
  • Citroen (Berlingo)
  • Dodge
  • Ford (Focus, Fusion)
  • Isuzu
  • Jeep
  • Peugeot (308, Partner)
  • Saab (9-3)
  • Skoda
  • Volkswagen (Jetta, Polo)

Fault code P0686 can sometimes be found with other errors. The most common are the following: P0685, P0687, P0688, P0689, P0690.

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