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.
What P25C9 means
P25C9 is a generic OBD-II chassis code in the ABS & brakes group.
- Check engine light
- Usually on
- Emissions-related
- No
- Limp mode possible
- Not typical
- Drive cycle to clear
- Not required
Get it fixed
Post this code once and get quotes from local mechanics. Free to post.
Makes the repair and parts links below specific to your car. Kept in this browser only.
Common causes
- 01Active DTC stored in the secondary brake moduleMost likely
- 02Communication loss interpreted as MIL requestLess likely
Can I drive with P25C9?
Drive carefully and get it checked soon.
Normal hydraulic braking usually still works, but ABS, stability control or brake assist may be off. Leave extra stopping distance, especially on wet or icy roads. If the pedal feels soft or the red brake light is on, stop driving.
Why we rate it urgent
- ABS or brake assist may be disabled
- Turns on the check engine light
Severity is our rule-based rating from the code’s flags in OBDex and the system it affects, not an inspection of your car. If the check engine light is flashing, or you notice overheating, low oil pressure, or brake or steering problems, stop driving and get help.
How to diagnose P25C9
- 1
Confirm the code and save freeze-frame data
Read the codes with an OBD-II scan tool and note every code, not just P25C9. Save the freeze-frame data (speed, load, temperature when the fault set) before clearing anything; it tells you when the fault happens.
- 2
Check for related codes first
Look for P25A2. Related codes often share a cause; a circuit or sensor code usually needs fixing before a performance code like this one.
- 3
Check live data
Watch the related sensor and actuator readings in live data (Brake control module B, Communication bus) and compare them with expected values in the service information for your vehicle.
- 4
Test the likely causes in order
Start with the most likely cause and work down: Active DTC stored in the secondary brake module; communication loss interpreted as MIL request. Test before replacing parts.
- 5
Check for recalls and service bulletins
Look up recalls for your exact vehicle with its VIN, and ask a dealer about technical service bulletins (TSBs). Manufacturers sometimes issue software updates or extended warranties for known faults.
- 6
Clear the code and verify the repair
After the repair, clear the code and drive the car under the conditions in the freeze-frame data to confirm it does not return.
P25C9 repair cost estimate
Estimate, not a quote. Labor times are typical ranges for this code from OBDex, and $125/hr is an assumed rate. Shop rates and labor times vary by vehicle and region. Diagnosis time may be billed separately.
- DIY difficulty
- Moderate
- Good DIY job?
- Usually a shop job
Needs a scan tool and some experience with testing.
Parts and systems involved
- Brake control module B
- Communication bus
P25C9 FAQ
What does P25C9 mean?
P25C9 means “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.
Is P25C9 serious? Can I keep driving?
We rate P25C9 as urgent severity. Drive carefully and get it checked soon. Normal hydraulic braking usually still works, but ABS, stability control or brake assist may be off. Leave extra stopping distance, especially on wet or icy roads. If the pedal feels soft or the red brake light is on, stop driving.
What causes P25C9?
The most common causes, from most to least likely: Active DTC stored in the secondary brake module; Communication loss interpreted as MIL request. Test before replacing parts, because several causes can set the same code.
How much does it cost to fix P25C9?
Typical labor is about $65–$315 at an assumed $125/hour shop rate, plus parts. That is an estimate, not a quote: labor times and rates vary by vehicle and region, and diagnosis may be billed separately.
Can I fix P25C9 myself?
DIY difficulty is moderate. Most owners leave this to a shop because it needs involved testing or specialist tools. Always confirm the cause before buying parts.
Will P25C9 clear itself?
If the fault was intermittent, the warning may go off once the computer stops seeing it, but the code stays in memory. Fix the cause; clearing the code alone doesn’t repair anything.