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P0596Cruise Control Servo Control Circuit High

The cruise control servo control circuit is stuck at a higher voltage than expected, indicating a short to battery voltage or an open being held high, and cruise control has been disabled.

Definition

While driving the cruise servo actuator, the module compares the control circuit's feedback voltage to the commanded state. P0596 sets when the circuit stays high when the actuator is commanded on, which points to a short to a voltage source, or an open circuit or failed winding that leaves the module's pull-up voltage on the line. The module disables cruise operation for the remainder of the ignition cycle when the fault is active, so the symptom is cruise that will not engage or drops out. There is no effect on normal engine driveability, and the MIL typically does not illuminate.

Common causes

  • Servo control wire shorted to a battery or ignition-voltage circuit
  • Open control wire or terminal held high by the module's internal pull-up
  • Open winding or clutch coil inside the servo actuator
  • Corroded, high-resistance terminal at the servo or module connector
  • Failed output driver in the cruise or engine control module

Common misdiagnoses

  • Servo replaced when the short to voltage is in a shared harness section
  • Control module replaced without checking for stray voltage on the circuit
  • Brake switch replaced because cruise disengages or will not set
  • Throttle cable replaced for an electrical circuit fault

Troubleshooting steps

  1. 1.Check for stray voltage

    Disconnect the control module and probe the servo control wire; any voltage present with the module out of circuit proves a short to a powered wire.

  2. 2.Measure the actuator circuits

    Check each servo winding for the specified resistance; an OL reading confirms an open that leaves the pull-up voltage on the circuit.

  3. 3.Verify wire and terminal integrity

    Confirm end-to-end continuity on the control wire and proper terminal tension at both connectors using an appropriately sized test pin.

  4. 4.Inspect shared harness runs

    Look for melted or rubbed insulation where the servo harness bundles with powered circuits near heat sources and moving parts.

  5. 5.Retest under command

    Run the scan tool servo output test and confirm the circuit voltage now follows the command, then complete a road test to verify cruise sets and holds.

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