Powertrain, generic (extended)

P25C9 Brake System Control Module B Requested MIL Illumination

Check engine lightOn

EmissionsRelevant

Drive cycleNot required

Limp modeNo

P25C9 tells the engine module that a second brake system control module, labelled B, has found an emission-relevant malfunction and wants the check engine lamp switched on. GM uses the code in its hybrid OBD tables 16OBDG01, 18OBDG01 and 19OBDG01: the engine module listens to the lamp request message from module B continuously, starting three seconds after power-up, and logs P25C9 as Type A but without its own lamp request, since the brake module already asks for the lamp. Which fault lies behind it only module B can say, so its memory is read first; a low battery or network trouble can also upset it. Brake module A reports the same situation as P25A2. As braking functions may be involved, the car should be checked soon.

Likely causes

Fault stored in brake system control module B

Weak battery or low system voltage

Communication fault between module B and its partners

Symptoms

Check engine lamp on
Brake or stability warnings at the same time

Repair estimate

DifficultyHardWorkshop job

Parts and labour€50–1,500

Time0.5–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

brake control module bcommunication bus

Related codes

P25A2 P25AF

References: gsi.ext.gm.com, gsi.ext.gm.com, gsi.ext.gm.com, law.cornell.edu.