P25C9
Brake System Control Module B Requested MIL Illumination
A second brake system control module (typically the redundant secondary unit on by-wire or AV-grade brake systems) has signalled an emissions-relevant fault and requested the engine controller to switch on the MIL.
- Check engine light: Yes
- Emissions related: No
- Repair difficulty: medium
Common causes
Ordered by how often each turns out to be the cause.
- Active DTC stored in the secondary brake moduleCommon
- Communication loss interpreted as MIL requestLess likely
Parts involved
- Brake Control Module B
- Communication Bus
P25C9 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.