Airbag control unit with part number A204 901 35 02

What is the function of the airbag control unit A204 901 35 02?

The airbag control unit A204 901 35 02 is the central control module for the entire occupant safety system (SRS) in the Mercedes-Benz C-Class, model series 204. It processes signals from crash sensors, deploys airbags and seatbelt pretensioners in the event of a collision, and stores relevant crash data in its internal memory.

As the brain of the safety system, this module—also known as the SRS control unit—is responsible for monitoring all connected components, such as airbags, seatbelt pretensioners, seat occupancy mats, and sensors. It continuously performs self-tests to ensure the system’s functionality.

When a fault is detected or after a crash, the airbag warning light on the instrument cluster illuminates to alert the driver that the restraint system may no longer be operational. The control unit is specifically tailored to the vehicle architecture of the C-Class models W204, S204, and C204.

Why does the A204 901 35 02 control unit fail?

The most common causes of failure for the A204 901 35 02 control unit are electronic malfunctions that can occur even without a crash. These include short circuits, undervoltage issues, or internal hardware defects on the circuit board. Another major cause is the storage of crash data following an impact, which can lock the device.

Electronic faults often result in complete system failure. Causes may include voltage spikes, a defective battery or alternator, or physical damage such as cable breaks or corrosion caused by fluids. Wiring beneath the seats is particularly susceptible to damage.

A frequent issue requiring repair is an internal, non‑erasable hardware defect, such as the one indicated by error code B220800. In these cases, electronic components like microcontrollers directly mounted on the circuit board are affected and must be professionally repaired.

For the C-Class W204, related part numbers also exist, including A2049012604, A2048706826, A2049012704, or the manufacturer’s number 5WK44740, which may exhibit similar fault patterns.

How do you recognize a fault in the A204 901 35 02?

A malfunction of the A204 901 35 02 control unit primarily manifests as a permanently illuminated airbag warning light (SRS lamp). Additional clear indicators include loss of communication with the OBD diagnostic tool and the occurrence of non‑erasable internal errors, even when no accident has occurred.

The most noticeable symptom is the continuously lit SRS indicator on the dashboard. This poses a critical safety risk, as in case of a fault, the entire airbag system is disabled and will not deploy during a crash. Furthermore, a vehicle with a glowing airbag light will fail the main inspection (TÜV).

Another typical problem is complete communication failure. Workshops can no longer access the control unit via the OBD port to read or clear fault codes. In some cases, even the vehicle identification number (VIN) stored in the control unit is lost.

Often, specific, persistent error codes remain stored and cannot be cleared. These include code B220800 for a severe hardware defect, or errors referencing components such as the driver’s airbag or side sensors—even if those parts have already been replaced.

In which vehicles is the A204 901 35 02 control unit installed?

The airbag control unit with part number A204 901 35 02 is specifically designed for various models of the Mercedes-Benz C-Class, model series 204. At Airbag24, we repair this control unit for the sedan (W204), the estate (S204), and the coupé (C204) of this vehicle generation.

At Airbag24, the A204 901 35 02 control unit is repaired for the following Mercedes-Benz models:

Mercedes-Benz C-Class W204 Sedan (2007–2014)

Mercedes-Benz C-Class S204 Estate (2007–2014)

Mercedes-Benz C-Class C204 Coupé (2011–2015)

How is the A204 901 35 02 repaired?

Repair at Airbag24 follows a standardized process: Upon receipt, the control unit undergoes a detailed hardware analysis. Defective electronic components are then replaced, and stored crash data or internal errors are erased using specialized software. Two years' statutory liability for defects applies to the repair.

For targeted diagnostics, it is recommended to read out the vehicle’s fault memory before removing the control unit and include the printout with your package. In the C-Class, the unit is typically located beneath the driver’s seat.

After arrival at Airbag24, our technicians perform a comprehensive hardware-level inspection. The repair includes replacing faulty components such as microcontrollers and addressing cold solder joints or damaged traces on the circuit board.

A key step is resetting the internal memory. Even if OBD communication is no longer possible, we can completely erase stored crash data and persistent fault codes, restoring the control unit to full functionality.

After installing the repaired unit in the vehicle, the vehicle’s fault memory must be cleared as a final step. The repaired unit returns with all vehicle data and is ready for immediate use — no coding or programming is necessary.