P0594 OBD-II Trouble Code: Cruise Control Servo Control Circuit/Open

P0594 code definition

The P0594 code indicates that there has been a malfunction in the electrical components of the cruise control servo control circuit, as detected by the powertrain control module, or PCM.

What the P0594 code means

This code means there is a problem with the cruise control circuit. If the PCM detects an issue with the cruise control, it will run a self-instituted test of the system. If there is an abnormal result for the voltage/resistance level within the cruise control servo control circuit, it will store the OBD-II generic code P0594.

What causes the P0594 code?

The most likely cause of an abnormal voltage/resistance level in the cruise control servo control circuit is a servo control unit that has failed, particularly in older model vehicles that integrate the servo with the cruise control module.

Other causes may include:

• Short circuits or open circuits in the cruise control buttons
• Shorted or open wiring
• Liquid spills or dirt/debris build up on the cruise control components
• Damaged connectors within the cruise control system
• Blown fuses, which may indicate a more serious issue

What are the symptoms of the P0594 code?

Before the P0594 code is detected, the cruise control will usually stop working. In most cases, 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 P0594 code?

After an OBD-II scanner detects the P0594 code, a technician should begin with a visual inspection of all the wiring, connectors, and other components of the cruise control system. Any damaged elements should be replaced, reconnected, or repaired as necessary. 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.

Common mistakes when diagnosing the P0594 code

When diagnosing the underlying cause of the P0594 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 clear the code and do a road test after each potential solution is attempted. If the most common repairs do not stop the Check Engine Light from reappearing, it can be a sign of an intermittent issue that requires an in-depth inspection and servicing.

How serious is the P0594 code?

If the P0594 code is being detected, a vehicle will still be drivable. The cruise control may not be usable, but this should not affect any other function of the vehicle. However, if a fuse is blown, there may be some other minor electrical issues, such as the inability to use the radio, interior lights that will not work properly, or the inability to use the turn signal. In some cases, such as the loss of a turn signal, the car would not be safe to operate.

What repairs can fix the P0594 code?

There are many ways that a technician can address a P0594 code detection. They include:

• Verify the code with an OBD-II scanner.
• Inspect and repair the electrical components related to the cruise control servo control system.
• Clear the code and perform a road test to ensure that the repairs solved the issue.
• Check and replace any vacuum supply hoses in a vacuum operated cruise control system.
• Disconnect the PCM, then inspect and test all wiring and switches in the cruise control system. Repair damaged circuitry as necessary.
• Replace cruise control switches.

Additional comments for consideration regarding the P0594 code

If a vehicle displays an unresolved P0594 code, or the code is still being stored in the PCM memory, the vehicle may not be able to pass the OBD 2 emissions test.

Need help with a P0594 code?

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