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P0687 OBD-II Trouble Code: ECM/PCM Power Relay Control Circuit High

Check Engine Light

P0687 code definition

A P0687 code is stored when the PCM (powertrain control module) detects an incorrect, abnormal, or erratic voltage reading from the relay control circuit that supplies it with its power.

What the P0687 code means

In most cases, when the PCM stores a P0687 code, it’s because it has detected overly high voltage from its power supply. When everything is working normally, the PCM receives a steady flow of power from the battery through a contact relay. If the voltage coming from the battery through this relay is too high, the PCM will store the P0687 code and illuminate the Check Engine light.

What causes the P0687 code?

Some of the most common causes for a P0687 code are:

  • An overcharging alternator
  • A defective ignition switch
  • A damaged or defective PCM power relay
  • A shorted or open circuit between the PCM and the PCM power relay

What are the symptoms of the P0687 code?

In most cases, the illuminated Check Engine light will be the only symptom for the P0687 code. In some instances, though, a no-start condition will occur, preventing the vehicle from starting in order to protect the PCM from damage.

How does a mechanic diagnose the P0687 code?

After identifying the P0687 code with an OBD-II scanner, the mechanic will begin by inspecting the wiring and connectors between the battery and the PCM. If there is evidence of damage, corrosion, a short, or an open circuit, they will repair the issue and then clear the code and attempt to start the vehicle again to check if the code is set once more.

If the code still sets after this, they will inspect and test the alternator’s performance, the ignition switch, and all other components and wiring related to the PCM power relay. After each test and repair, they will clear the codes and retest the system.

Common mistakes when diagnosing the P0687 code

If you’re in too much of a rush to diagnose the problem, you may believe that the PCM needs to be replaced to allow the vehicle to start again. Doing this without fixing the actual underlying cause of the P0687 code will be expensive and unnecessary. With a thorough inspection and diagnosis, you may be able to save a lot of time, energy, and money.

How serious is the P0687 code?

If the P0687 code results in a no-start condition, then the problem will need to be resolved before the vehicle is drivable. Even if there isn’t a no-start condition and the vehicle can be started and driven, though, it’s important to note that too much voltage sent to the PCM can severely damage it. The longer you go without resolving the problem, the more likely you are to need an entirely new PCM to fix it.

What repairs can fix the P0687 code?

In many cases, the problem can be resolved by performing one of the following repairs:

  • Repairing or replacing the alternator and/or its wiring and connectors.
  • Replacing the ignition switch.
  • Replacing the PCM power relay.
  • Repairing or replacing any faulty wiring or connectors between the battery, the PCM power relay, and the PCM.

Additional comments for consideration regarding the P0687 code

If the PCM receives too much voltage or its power supply is erratic, a number of other codes and symptoms can begin to appear. It’s important to note the order in which these codes were stored by the PCM, as this can tell you whether or not they’re related. Don’t just try to resolve the symptoms before thoroughly inspecting the problem, and remember that the PCM may not actually need to be replaced, even if it is malfunctioning and throwing several codes. Replacing a faulty PCM power relay could be the solution to the problem, and it could save you a great deal of money and frustration.

Need help with a P0687 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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