P25C9: The brake controller has requested an emissions warning

P25C9 is brake system control module B requested MIL illumination. The 2018 GM ECM common table describes the ECM monitoring that module's warning request when it has detected an emissions-related fault.

The request is not the underlying diagnosis

The monitored condition combines an emissions-related DTC in brake control module B with its request for MIL illumination. The ECM checks this continuously after at least three seconds since power-up. P25C9 therefore points to information held by another controller; it does not identify a brake pressure sensor, hydraulic component or ECM part that needs replacement.

The source's reporting column labels this row “Type A, No MIL,” despite the request terminology. That is a reason not to infer the dashboard lamp's exact behavior from this row alone. The clear supported finding is receipt of the other module's fault request.

What to retrieve next

A full module scan should preserve the originating brake-controller DTCs and their current/history status. Clearing only the ECM's request code can remove a useful pointer without resolving the initiating fault. An ECM with no local brake measurements cannot substitute this request for those details.

Follow the service procedure for the actual underlying code and then confirm the request state clears appropriately. Because the originating system controls braking, any functional brake concern needs professional assessment; P25C9 alone supplies neither a safe-driving conclusion nor a brake repair procedure.

Source and applicability

This explanation follows the documented GM monitor, not a verified application to every vehicle with this code. Confirm the vehicle, reporting module and calibration in current service information. GM diagnostic table, physical page 466.