P0591

Universal

Cruise Control Multi-Function Input B Circuit Range/Performance

Definition source: General OBD-II reference catalog
Definition verification: Not independently verified for display.
Time: Varies by diagnosis and vehicle | Diagnostic difficulty: Medium
Low Severity

The vehicle’s control system has detected that the cruise-control multi-function “B” input signal is outside its expected range or is not behaving as expected. This input may come from cruise-control switches or related wiring, depending on the vehicle. Cruise control may be unavailable or work intermittently; the code alone does not identify a failed part.

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⚠️ Before You Begin (Safety First)

If cruise control behaves unpredictably or will not disengage, cancel it using the vehicle’s controls or brake, and stop safely if needed. Avoid using cruise control until the issue is diagnosed. If the vehicle otherwise drives normally and cruise control is off, this fault usually does not require stopping immediately.

Common Symptoms

  • Cruise control may not set or may be unavailable if the control system rejects the input signal.
  • Cruise-control buttons may respond intermittently if the input signal varies or is inconsistent.
  • The cruise-control indicator or a warning light may appear, depending on the vehicle.
  • There may be no noticeable symptoms if the fault does not affect other vehicle functions.

Common Causes

  • A worn, damaged, or contaminated cruise-control switch may produce an input outside the expected range.
  • A loose, corroded, damaged, or poorly connected wire or connector may interrupt or alter the signal.
  • A short or open in the relevant input circuit may prevent the control system from reading the input correctly.
  • A related steering-wheel wiring component may be involved on vehicles whose circuit passes through that assembly.
  • A control module or its connection may be at fault, but should be considered only after circuit and switch checks.

Diagnostic Steps

  1. Check the owner’s manual to confirm how cruise control should be enabled and canceled, then note whether the fault is constant or intermittent.
  2. With the vehicle parked and cruise control off, visually check accessible cruise-control buttons and switches for damage, sticking, or contamination.
  3. Inspect only accessible wiring and connectors for looseness or visible damage; do not dismantle the steering column or disturb airbag-related parts.
  4. If you have a suitable scan tool, record the code and any available cruise-control input data before clearing codes; compare the reported input with button operation if the tool supports it.
  5. Have a qualified technician perform circuit tests or steering-wheel assembly checks if the fault remains, since exact test points and procedures depend on the vehicle.

When to Call a Professional

Use vehicle-specific service information to check the cruise-control input data and compare switch operation with the control module’s reported state. Inspect the relevant wiring and connectors, then test the circuit for unwanted resistance, opens, or shorts using the specified procedure. Consider the switch, wiring, and module only after testing supports that conclusion; steering-wheel wiring components apply only on vehicles whose circuit uses them.

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Commonly Asked Questions

Does P0591 prove the cruise-control switch has failed?

No. It identifies an out-of-range or poorly performing input circuit, not a specific failed part. The switch, wiring, connectors, or another part of the input path may need testing.

Can I drive with P0591?

If the vehicle drives normally and cruise control is off, the fault is generally limited to cruise-control operation. Do not use cruise control if it is unreliable, and stop safely if it will not disengage.

Why might the code appear when the buttons seem to work?

The signal can be intermittent or outside the expected range even if a button sometimes responds. Scan data and circuit checks can help distinguish a switch issue from a wiring or connection problem.

Should I replace the switch before testing the circuit?

No. Inspect and test the relevant switch, wiring, and connections using vehicle-specific service information before replacing parts.

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