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P0578 OBD-II Trouble Code: Cruise Control Multi-Function Input "A" Circuit Stuck

Check Engine Light

P0578 code definition

The P0578 code is detected in instances where the PCM registered a malfunction with the cruise control multi-function input circuit.

What the P0578 code means

In order to control vehicle speed when the cruise control is activated, the cruise control system works with the PCM to monitor the operation. If the PCM detects that the vehicle speed is not being automatically controlled by the cruise control, it will initiate a self-test of the system. If, during this test, an abnormal voltage reading is detected in the cruise control multi-function input circuit, the PCM will store the P0578 code.

What causes the P0578 code?

In most cases, this code is caused by a failed cruise control switch, especially if the cruise control button has had liquid spilled on it, or if dust or debris has been allowed to build up on the buttons. Other causes may include:

  • Shorted, damaged, or open wiring, connectors, or wiring
  • Blown fuses, which may indicate a more serious issue

What are the symptoms of the P0578 code?

In most cases, the cruise control will not work, and the Check Engine light will activate within two to three drive cycles after the PCM has detected the issue.

How does a mechanic diagnose the P0578 code?

After an OBD-II scanner detects the P0578 code, a technician will perform visual inspection of all the wiring, connectors, and other electrical components of the cruise control system. After repairing or replacing any components, they should then clear the code and perform a test drive, checking that the cruise control works as it should, and that the Check Engine light does not reappear. This process should be repeated after each attempted repair, to ensure that no unnecessary repairs are performed. If this process does not fix the issue, it is likely that the cruise control switch is faulty, and needs to be replaced.

Common mistakes when diagnosing the P0578 code

When diagnosing the underlying cause of the P0578 code, the most common mistake is forgetting to disconnect the PCM and the cruise control module when testing circuit resistance. It is also important to check for blown fuses before replacing any parts – a blown fuse will cause a new replacement part to fail just as the old one did.

How serious is the P0578 code?

If the P0578 code is not considered an emergency that must be fixed right away. The vehicle will still be operable, and only the cruise control function is likely to be interrupted. However, if a blown fuse is part of the problem, that may cause other electrical issues that can lead to unsafe driving conditions – such as losing the ability to activate the turn signal. In this case, the P0578 code would need to be addressed right away.

What repairs can fix the P0578 code?

In order to repair the P0578 code, a mechanic may replace the cruise control switch, repair or replace any damaged electrical components, and/or replace any burnt fuses.

Additional comments for consideration regarding the P0578 code

While the P0578 code is stored in the PCM, the vehicle may not be able to pass an OBD-II emissions test. In states where emissions tests are required, this could mean that the vehicle will not be legally registered for use on public roadways.

Need help with a P0578 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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