P0578Cruise Control Multi-Function Input "A" Circuit Stuck
The multiplexed cruise control switch signal is stuck at one value, as if a button is being held down continuously.
Definition
Many vehicles run all cruise buttons through a single multi-function input, where each button switches a different resistance and produces a distinct voltage. P0578 sets when the module sees that signal frozen at one button's value for longer than plausible — a stuck button or a shorted circuit. The module disables cruise control as a failsafe, since a stuck set or resume command could otherwise cause unintended acceleration. This code is common on GM vehicles with steering-wheel switch or clockspring failures; the MIL stays off and engine operation is unaffected.
Common causes
- Physically stuck or worn cruise switch button (most common)
- Liquid spilled into the steering-wheel switch pad holding a contact closed
- Shorted clockspring circuit mimicking a pressed button
- Chafed harness shorting the signal wire to ground or another circuit at a fixed resistance
- Internally failed switch resistor ladder
- Faulty module input (rare)
Common misdiagnoses
- Replacing the brake light switch
- Replacing the PCM or body control module before testing the switch
- Replacing the cruise actuator/servo
- Replacing the entire steering wheel when only the switch pad or clockspring is at fault
Troubleshooting steps
1.Read the cruise switch status on a scan tool
Note which button the module reports as stuck with your hands off the wheel; that identifies the offending circuit step.
2.Measure the signal voltage at rest
Back-probe the multi-function input; a voltage matching one button's step with nothing pressed confirms the stuck condition.
3.Disconnect the switch to isolate it
If the signal returns to its at-rest value with the switch unplugged, replace the switch; if not, the fault is in the clockspring or harness.
4.Test the clockspring
Check the spiral cable circuits for shorts between conductors while rotating the wheel through its full travel.
5.Verify the repair
Confirm every button produces its correct, momentary voltage step, clear the code, and road test cruise operation.