P0578 indicates that the cruise control multi‑function input circuit is stuck in an on or off position. The PCM has detected an abnormal voltage condition on the cruise control switch or related wiring.
The switch can develop internal shorts or opens, causing the input circuit to read a constant voltage.
Wire harnesses that route to the cruise control module can become cracked, corroded, or have poor connections, creating a stuck circuit condition.
A short in the harness or a grounding issue can force the signal line to a constant voltage level.
The electronic cruise control module or its actuator may have an internal fault that pulls the input line low or high.
Only after confirming all wiring and component issues should a PCM defect be considered, as it is an uncommon cause.
Replacing the PCM is rarely the correct solution. In most cases the problem is a faulty switch, wiring issue, or a bad cruise control module. PCM replacement should only be considered after exhaustive testing of those components.
The PCM will monitor the cruise control input circuit continuously. If the underlying fault remains, the code will set again and the light will stay on.
The vehicle can be driven, but cruise control will be unavailable and the underlying fault could affect other functions that share the same circuit. It is advisable to diagnose and repair the issue promptly.
If the switch is merely dirty or corroded, cleaning may restore proper operation. However, if the switch has internal damage or a short, it will need to be replaced.