On commercial vehicles that use air brakes and have ABS (Anti-Lock Braking Systems), if the ABS bulb stays on after the bulb test or comes on during operation, it can mean:

Question

Quizplus · Accepted Answer

After completing the ABS bulb check (to ensure the ABS bulb is actually in working order), the bulb should go out and remain out. If it stays on or if it comes on during operation of the vehicle, it can mean that the vehicle has lost ABS control on one or more wheels. When this occurs, drivers should report the condition and have the vehicle serviced as soon as possible.

On Commercial Vehicles That Use Air Brakes and Have ABS