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P0591Cruise Control Multi-Function Input "B" Circuit Range/Performance

The cruise control Multi-Function Input B signal is within electrical limits but its value or behavior is implausible, such as voltages that do not match any valid button step.

Definition

The module validates the Multi-Function Input B resistor-ladder signal not just for opens and shorts but for rationality: every reading must land on a defined button-state voltage and transition cleanly. P0591 sets when the signal wanders between steps, lands on undefined intermediate values, or changes in ways that do not match possible button operation. This usually reflects a degrading switch or a high-resistance connection rather than a hard circuit failure, and cruise operation may be intermittent, erratic, or fully disabled while the fault is active. There is no impact on engine operation and the MIL typically stays off.

Common causes

  • Worn cruise switch with drifting or unstable ladder resistances
  • High-resistance connection at the clockspring or column connector shifting all voltages
  • Partial corrosion inside the switch from moisture or spills
  • Intermittent chafe on the signal wire touching ground or another circuit
  • Marginal terminal tension causing voltage flicker over bumps

Common misdiagnoses

  • Clockspring replaced when added connector resistance is the true cause
  • Control module replaced for an unstable but external signal
  • Cruise servo replaced for an input-rationality fault
  • Switch replaced twice because the added harness resistance was never found

Troubleshooting steps

  1. 1.Record the signal while operating the buttons

    Graph the Input B voltage on a scan tool or scope while pressing each button; every press should snap to its charted voltage without drift or intermediate values.

  2. 2.Compare readings at switch and module

    Measure the same button-press voltage at the switch connector and again at the module pin; a difference indicates resistance added by the clockspring or harness.

  3. 3.Wiggle-test the column wiring

    Watch the live voltage while turning the wheel and flexing column connectors to reproduce the erratic behavior.

  4. 4.Measure switch steps precisely

    Check each ladder resistance with the pod disconnected and reject the switch if any step is outside the service-data tolerance or unstable while held.

  5. 5.Clean and re-tension terminals

    Address any corroded or loose terminals found, then retest all functions through a complete drive cycle before releasing the vehicle.

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