P0686 OBD-II Trouble Code: ECM/PCM Power Relay Control Circuit Low

Check Engine Light

P0686 code definition

The P0686 trouble code detects a voltage error with the ECM/PCM power relay control circuit.

What the P0686 code means

The P0868 code is a standard OBD-II trouble code that notes a voltage error with the relay control circuit that is responsible for supplying the powertrain control module (PCM) and engine control module (ECM) with power. The relays send an output signal to the PCM, and rely on a ground signal, battery voltage signal, and ignition switch input signal. Any time that the PCM notes that the voltage level is abnormal relative to the manufacturer’s recommendations, the P0686 code will be triggered.

What causes the P0686 code?

Many things can cause the P0686 trouble code to be detected:

  • Dead battery
  • Defective circuits
  • Defective PCM relay
  • Malfunctioning PCM
  • Defective battery cables or cable ends
  • Damaged or defective electrical components in the PCM, such as fuses, wires, and connectors

What are the symptoms of the P0686 code?

The P0686 trouble code will usually be accompanied by the Check Engine Light illuminating on the vehicle’s instrument cluster. It is also common for the engine to not start, or not crank.

How does a mechanic diagnose the P0686 code?

The P0686 trouble code should be assessed with the help of a basic OBD-II trouble code scanner. A trained technician will use the freeze frame data from the scanner to observe the code, and to look for additional trouble codes that are stored. Trouble codes should always be cared for in the order that they appear in. the mechanic should reset the trouble codes and restart the vehicle, to see if the code returns. If the P0686 code does not return, then it is likely an intermittent error, or perhaps the result of an erroneous trigger.

If the trouble code returns, then the mechanic should begin with a visual inspection of the electrical components in the PCM. All fuses, wires, and connectors should be inspected, and replaced if damaged. The mechanic should then test to see if the Check Engine Light illuminates when the key is turned to the engine off position. If the light does not illuminate, then the PCM relay is likely the issue, and will need replacement.

If this is not the issue, the mechanic will visually inspect the battery cables and cable ends to make sure that they are all in working order. Next, the charge of the battery will be tested, and a battery load charging system test should take place, with the results being compared to manufacturer’s recommended results. Next, the voltage and ground signals of the applicable circuits will be tested and assessed.

After any repairs, the trouble codes will need to once more be reset, and the vehicle again restarted. The mechanic will then look to see if the P0686 code returns. This allows the mechanic to know as soon as the issue has been resolved.

Common mistakes when diagnosing the P0686 code

The most frequently made mistake when diagnosing the P0686 trouble code is the failure to properly follow the OBD-II trouble code diagnosis protocol. The protocol should be followed step-by-step at all times to guarantee an efficient and thorough inspection and repair.

It is common for the PCM to be replaced erroneously, without even checking the PCM relay, which is a much more likely culprit, and a quicker and more affordable fix.

How serious is the P0686 code?

The P0686 trouble code may not impact the drivability of a vehicle, but it also may leave the car unable to turn on. The code should be addressed as soon as it is detected.

What repairs can fix the P0686 code?

Some possible repairs for the P0686 trouble code are:

Additional comments for consideration regarding the P0686 code

Not all vehicles use a PCM/ECM relay to power the PCM. Some vehicles instead rely on an ignition switch or battery block wire. Vehicles that do not have a PCM/ECM relay will not have a P0686 trouble code.

It is rare for the PCM to need to be replaced, so it should not be considered the issue unless all other options are checked. If the PCM does require replacement, it will need to be reprogrammed.

Need help with a P0686 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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