What it means
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.
At a glance
- Usually lights the check-engine light
How it works
The second brake system control module monitors its brake-control functions and can ask the engine controller to turn on the MIL when it detects a fault. P25C9 means that request was received, or that a communication problem was interpreted as a request; the code alone does not identify the underlying brake fault.
Common causes
Most likely first.
- Active DTC stored in the secondary brake module
- Communication loss interpreted as MIL request
Symptoms
- Brake system warning light may illuminate
- Check-engine light may illuminate
How to diagnose it
- Scan all modules and record freeze-frame data and codes before clearing anything, paying particular attention to faults stored by the secondary brake module.
- Check whether the brake warning light is on and note any changes in braking feel, since those clues help assess whether the module's request relates to an active brake-system problem.
- Inspect the secondary brake module's connectors, power and ground connections, and accessible wiring for looseness, corrosion, damage, or water intrusion.
- Use a scan tool that can access the brake modules to read their status and communication data and identify whether a specific fault or lost communication prompted the request.
- If communication faults are present, check the relevant network wiring and module power and ground circuits using the vehicle's wiring information and manufacturer's specifications.
Common fixes
- Repair a loose, corroded, or damaged secondary brake-module connector or wire.
- Restore a faulty power or ground connection to the module.
- Repair a network wiring fault if communication loss triggered the request.
- Address the underlying fault stored in the secondary brake module, guided by its specific diagnostic codes.
Can I keep driving?
If the brake warning light is on, braking feels different, or the vehicle reports a brake-system fault, stop driving when safe and have the brake system assessed; a fault may affect braking assistance or control. If braking feels normal and only the check-engine light is on, drive cautiously only as needed to reach a safe place for diagnosis.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.