C0420
Adaptive Cruise Control Module Internal Performance
The adaptive cruise control module has reported an internal performance fault — for example self-test failure, supply voltage out of range or invalid configuration. ACC is disabled.
- Check engine light: Not always
- Emissions related: No
- Repair difficulty: hard
Symptoms
- ABS warning light on
- ESC/stability control deactivated
- Possible reduced brake assist
- Speedometer may behave erratically
Common causes
Ordered by how often each turns out to be the cause.
- ACC module internal faultCommon
- Supply voltage out of rangePossible
- Module configuration invalid after part exchangeLess likely
Parts involved
- Acc Module
- Vehicle Power Supply
- Communication Bus
C0420 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.