Faulty or worn cruise control switch or steering wheel/stalk control contacts; Wiring damage, chafing or poor connections in the multi-function input circuit; Corroded or loose connector at the switch, clockspring or control unit; Faulty or misadjusted brake or clutch pedal switch feeding the same input logic; Worn or damaged clockspring (steering wheel spiral cable) on models where the switch signal passes through it; Fault within the control unit that processes the input; Outdated control unit software where the manufacturer has issued a correction for this fault.

P0591

Cruise Control Multi-Function Input B Circuit Range/Performance.

Generic
Powertrain
Generic code
Low severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
5: AreaVehicle speed, idle control and auxiliary inputs
91: FaultThe specific fault in that area

What it means

P0591 is the generic code for a range or performance fault in the cruise control multi-function input B circuit. The control unit monitors the signal from the cruise control switch or control input and has seen a value outside the expected range, or one that does not match other inputs such as the brake or clutch switch status. How input B is wired and which switch it refers to varies by manufacturer and model.

Symptoms

Cruise control will not engage, or cancels itself unexpectedly
Cruise control buttons or stalk switch work intermittently or not at all
Cruise or engine management warning light on
Cruise control set speed is not held or drops out over bumps
Brake lights or other switch-related functions behaving oddly in some cases

Possible causes

Faulty or worn cruise control switch or steering wheel/stalk control contactslikely
Wiring damage, chafing or poor connections in the multi-function input circuitlikely
Corroded or loose connector at the switch, clockspring or control unitpossible
Faulty or misadjusted brake or clutch pedal switch feeding the same input logicpossible
Worn or damaged clockspring (steering wheel spiral cable) on models where the switch signal passes through itpossible
Fault within the control unit that processes the inputrare
Outdated control unit software where the manufacturer has issued a correction for this faultrare

How to diagnose

Read all stored codes and freeze frame data, noting whether other cruise, brake switch or steering wheel control codes are present
Test every cruise control button and see whether the fault is constant or intermittent; check live data for the switch input if available
Visually inspect the switch, wiring and connectors for damage, water ingress, corrosion or contamination
Check the brake and clutch pedal switches for correct operation, adjustment and brake light function
Wiggle-test the harness and connectors with live data displayed to find intermittent faults
Measure the switch circuit voltage and resistance in each switch position against the manufacturer's specification for that model
Check the circuit for open circuits, short circuits and high resistance between the switch and the control unit, including the clockspring where fitted
Check for applicable software updates or technical bulletins for the vehicle before considering the control unit

How to fix

Replace the faulty cruise control switch or steering wheel control assembly
Repair damaged wiring and clean or replace corroded or damaged connectors
Replace or correctly adjust the brake or clutch pedal switch
Replace the clockspring if testing shows it is the source of the signal fault
Update the control unit software if a manufacturer correction applies, or replace the unit only after all other causes are ruled out

Can I keep driving?

Low severity

The car is normally safe to drive as this affects cruise control only, so you can book it in at your convenience. If the brake lights are also not working correctly, get that checked promptly.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.