P0578 means the engine control module saw the cruise control multi-function input A held at a valid button value for longer than a person would press it. The related codes are P0579 (range or performance) and P0580 (low) for the same input, and P0581 through P0583 for a second input B. "Multi-function input" is the module's name for the single wire that carries all the cruise buttons as a resistor ladder.
Severity: low. Can you keep driving? Yes. Cruise control is disabled while the fault is present. Nothing else on the vehicle is affected, unless a blown fuse shared with turn signals or wipers is the underlying cause, in which case that fuse is the priority.
How a resistor-ladder switch gets "stuck"
Each button on the wheel places a different resistor between the signal wire and ground. The module reads the resulting voltage. Nothing pressed reads the pull-up voltage; each button reads its own step. A stuck code sets when the voltage sits on a step for many seconds or minutes.
Three things do that:
- A button held down mechanically: a cap that binds in its housing, a spring that has collapsed, or dried drink and grime under the button. Mitsubishi's service information for this code reads simply "cruise control switch stuck," and the inspection it calls for is mechanical.
- Contacts that stay closed electrically: corrosion bridging the contacts inside the switch.
- The signal wire shorted to ground, or to another wire, through a chafe in the column harness or a fault in the clockspring. A hard short to ground reads as the lowest step on many ladders, which is a valid button.
Causes, in the order they are found
A sticky or contaminated button on the steering wheel switch.
Corroded switch contacts holding one button closed.
A clockspring with conductors touching or broken, usually with the horn, radio buttons or airbag light acting up too.
A chafed column harness shorting the signal wire.
A blown fuse on vehicles where the switch and other column functions share a feed.
The module, last.
A diagnostic order
- Record the code and companions. An airbag code points at the clockspring.
- Press every button on the wheel and feel for one that does not return cleanly. Look under the caps for residue.
- Watch the cruise switch voltage live. With nothing pressed it should sit at the pull-up value. A value parked on a button step is the stuck input; press that button and see whether the value changes at all.
- Unplug the switch. If the live value jumps to the pull-up with the switch unplugged, the switch is the fault. If it stays parked, the wiring or clockspring is.
- Measure the switch at its connector: open with nothing pressed, each button's listed resistance when pressed.
- Check the clockspring and column harness for a short between the signal wire and ground or adjacent wires, with the airbag precautions followed.
- Clean or replace the switch, or repair the wire, clear the code, and verify cruise engages and cancels on a road test.
Frequently asked questions
Can I drive with a P0578 code?
Yes. Cruise control is disabled and the engine is otherwise normal. Check the fuse box if other column functions such as turn signals have stopped, because some vehicles share a fuse.
Is P0578 the cruise control switch?
Often. A button that binds or contacts that have corroded hold the input at one value. But a signal wire shorted to ground in the clockspring or column harness produces the same "stuck" reading. Unplug the switch and watch the live value: if it frees, the switch is the fault; if not, the wiring is.
Why did P0578 appear after a drink was spilled on the console or wheel?
Sugar residue under a button cap keeps it from returning, or bridges the contacts. Cleaning the switch often fixes it; a switch that has been wet long enough for the contacts to corrode is replaced.
How much does it cost to fix P0578?
Cleaning a switch costs little. A steering wheel switch is an inexpensive to moderate part. A clockspring is similar with about an hour of labor and airbag precautions. A harness repair is under an hour.
Related codes
- P0579 / P0580: multi-function input A range or performance, and low.
- P0581 to P0583: multi-function input B.
- P0565 / P0566: on and off signals; see P0565 and P0566.
- P0575: cruise control input circuit.
The code number itself tells you the family and the domain; how to read an OBD2 code number explains what each character means.