Mercedes-Benz W211 SBC brake warning

It is a fault message from the SBC brake system (Sensotronic, electrohydraulic brake). The cause can be a leak, air or a blockage in the brake circuit, the brake light switch, a weak auxiliary battery, or the wear counter of the SBC unit. The fault is read with a tester at a workshop.

The red warning indicates a real fault in the brake system that affects safety. The brakes are safety-critical, so it is best not to drive the car. If the pedal sinks to the floor or braking force is missing, the car is towed to a workshop.

A common cause is a leaking brake line, especially around the rear wheels under the protective plastic covers. Other causes are a faulty brake light switch, the sensor of a worn brake pad, the brake fluid reservoir sensor, or the SBC unit's operation counter filling up. Replacing the unit does not always remove the warning if the cause is elsewhere.

Workshops do not usually open the unit; a unit found faulty is replaced. Some faults have been covered under warranty, so it is worth asking a Mercedes-Benz dealer. Brake work is safety-critical, and a workshop should check the result.