Faulty cruise control switch or steering column stalk (internal open circuit or worn contacts); Damaged, chafed or corroded wiring in the multi-function input circuit (open circuit or short to voltage); Poor connection or corroded terminals at the switch connector or control unit connector; Worn clock spring or rotary connector in the steering column, on models where switches are routed through it; Fault in the brake or clutch pedal switch that shares or affects the cruise input circuit, depending on model; Faulty control unit (engine, body or cruise module) input circuit; Outdated or faulty control unit calibration where the manufacturer has issued a known fix.

P0593

Cruise Control Multi-Function Input B Circuit High.

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
93: FaultThe specific fault in that area

What it means

P0593 is the generic SAE J2012 code for Cruise Control Multi-Function Input B Circuit High. The control unit monitors the voltage on the cruise control multi-function input circuit (typically the switch or stalk signal) and has seen it higher than the expected range. This usually points to an open circuit, a short to voltage or a fault in the switch itself. The exact circuit layout and which switch is 'input B' depends on the make and model.

Symptoms

Cruise control will not engage or set
Cruise control cuts out unexpectedly while driving
Cruise control indicator light not working or flashing
Malfunction indicator lamp may be on
Steering wheel or stalk cruise buttons unresponsive
Other functions sharing the switch (such as speed limiter or adaptive cruise) unavailable

Possible causes

Faulty cruise control switch or steering column stalk (internal open circuit or worn contacts)likely
Damaged, chafed or corroded wiring in the multi-function input circuit (open circuit or short to voltage)likely
Poor connection or corroded terminals at the switch connector or control unit connectorlikely
Worn clock spring or rotary connector in the steering column, on models where switches are routed through itpossible
Fault in the brake or clutch pedal switch that shares or affects the cruise input circuit, depending on modelpossible
Faulty control unit (engine, body or cruise module) input circuitrare
Outdated or faulty control unit calibration where the manufacturer has issued a known fixrare

How to diagnose

Read the stored codes and freeze frame data, noting whether the fault is current or historic, and check for other cruise, brake switch or steering column related codes
Check the cruise control switch operation and the dashboard indicator, and test each button for response
Inspect the wiring, connectors and terminals between the switch and control unit for damage, chafing, moisture or corrosion
Check the brake and clutch pedal switches for correct operation and adjustment, as these can affect cruise availability
Using the wiring diagram for the model, measure the reference voltage and signal voltage at the switch connector with the switch in each position
Check the circuit for continuity and for short to voltage, with the connectors disconnected and the battery safely isolated as required
Check the clock spring or rotary connector for continuity if the switches are routed through the steering column
Check for applicable manufacturer service information or calibration updates before replacing the control unit

How to fix

Replace the faulty cruise control switch or steering column stalk
Repair or replace damaged wiring and clean or renew corroded connector terminals
Replace the clock spring or rotary connector if it is found faulty, following the safe airbag procedure for the model
Replace or adjust a faulty brake or clutch pedal switch
Update the control unit calibration only if a known outdated or faulty calibration is confirmed, or replace the control unit if its input circuit is proven faulty

Can I keep driving?

Low severity

The car is normally safe to drive, but the cruise control will probably be unavailable. Book it in for diagnosis, and take care if the brake switch is suspected as it may affect brake lights.

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