Mercedes-Benz W202 BAS/ASR and ESP lights on
Why is the Mercedes-Benz W202 BAS/ASR or ESP light on?
The most common cause is a faulty brake light switch, which gives the control unit a wrong signal. Other causes are the wheel speed sensors and their connectors, the lateral acceleration sensor, the steering angle sensor, a weak battery and, rarely, the BAS control unit. It is worth reading the fault codes first.
How is the brake light switch checked?
The switch has two circuits, one of which connects and the other breaks. The brake lights working therefore does not prove the switch is in good condition. The part varies by chassis number, so check the fitment. Measuring requires removing the footwell trim.
Does the light go out by itself or do the fault codes have to be cleared?
The codes are read and cleared with a fault code tester; disconnecting the battery does not clear them. If the light comes on again when the brake is pressed, the cause is still the brake light switch or its wiring. On ESP cars the steering angle sensor is calibrated by turning the steering wheel from lock to lock with the engine running.
What if the pedal is hard and braking is weak while the BAS light is on?
The cause may be a leak in the vacuum hose running from the intake manifold to the brake servo. Air in the brake system can also turn the light on. Brakes are safety-critical, so have a workshop check them and the result of the repair.