Mercedes-Benz W163 BAS/ESP light stays on
The W163 BAS/ESP light is on and won't go out, what causes it?
There are several causes: the brake light switch, relay K6, a brake fluid level that is too low, an incorrect reading from the steering angle sensor or a weak battery. Go through them one at a time. The brakes are safety-critical, so have a workshop check the result.
The light is on even though the brake light switch was replaced, what helps?
The brake light switch is a common cause, but replacing it alone is not always enough. Also check relay K6: if the relay has too low an amp rating, the light comes on again, and fitting the correct type of relay can fix the fault. A wheel speed sensor plug can also cause the same symptom.
Can the brake fluid level, steering angle sensor or battery turn on the BAS/ESP light?
Yes. A brake fluid level dropping to the minimum turns the lights on, and the cause must be found, for example a leaking brake caliper. The steering angle sensor can show a wrong value if the steering wheel is crooked after replacing joints; an alignment fixes it. A weak battery can cause phantom faults.