Worn, sticky or faulty cruise control switch or steering wheel/stalk control assembly; Contaminated switch (spilled drink, dirt or moisture) holding the contacts closed; Wiring short to voltage or ground, or chafed wiring in the circuit, including through the steering column clockspring where fitted; Corroded or loose connector at the switch or control unit; Faulty brake or clutch pedal switch feeding the same input on some models; Internal fault in the engine control unit or the module that reads the switch.

P0578

Cruise Control Multi-Function Input A Circuit Stuck.

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
78: FaultThe specific fault in that area

What it means

P0578 means the engine control unit has detected that the cruise control multi-function input A circuit is stuck, so the signal from the cruise control switch or its circuit does not change as expected. The control unit sees a switch state that appears to be held on or fixed at one value for longer than it should be. The cruise control is normally disabled while the fault is present, and the exact switch layout depends on the model.

Symptoms

Cruise control will not engage or cancels immediately
Cruise control light or warning lamp on the dash
Cruise control buttons on the steering wheel or stalk do not respond or behave oddly
Engine management light on in some vehicles
Cruise control set speed drops or cannot be resumed

Possible causes

Worn, sticky or faulty cruise control switch or steering wheel/stalk control assemblylikely
Contaminated switch (spilled drink, dirt or moisture) holding the contacts closedlikely
Wiring short to voltage or ground, or chafed wiring in the circuit, including through the steering column clockspring where fittedpossible
Corroded or loose connector at the switch or control unitpossible
Faulty brake or clutch pedal switch feeding the same input on some modelspossible
Internal fault in the engine control unit or the module that reads the switchrare

How to diagnose

Read the stored codes and freeze frame data, note whether other cruise, brake switch or steering wheel control codes are present, then check for any relevant technical bulletins for the model
Operate each cruise control button or stalk position and check for a stiff, sticking or non-returning action; look for visible contamination
Inspect the switch, its connector and the wiring for damage, moisture, corrosion or chafing, including any steering column clockspring connection
Using live data, watch the switch input status with the switch at rest and while pressed; a value that never changes or stays active points to a stuck input
Check the brake and clutch pedal switches for correct operation and adjustment, as some models share their signals with cruise control
Measure the circuit voltage and resistance against the manufacturer's wiring diagram and specification, with the switch disconnected and connected
Check the circuit for shorts to voltage or ground and for resistance at the connectors
If the circuit and switch test fine, check the control unit power, grounds and software status as specified for that model

How to fix

Replace the faulty cruise control switch or control assembly
Clean or dry the switch and connectors if contamination is the cause, and replace any parts that stay faulty
Repair or replace damaged wiring or connectors in the switch circuit
Replace or adjust a faulty brake or clutch pedal switch where it affects the input
Replace the clockspring if testing shows it is the cause on models that use one
Repair or replace the control unit, or update its software where an outdated version is a known cause for that model, only after the rest of the circuit is proven good

Can I keep driving?

Low severity

The car can normally be driven as usual, but cruise control will probably not work. Get it checked soon, especially if the brake switch could be involved, as that can affect brake lights.

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