Airbag control unit with part number 8A0 959 655 B

What is the function of the Airbag Control Unit 8A0 959 655 B?

The Airbag Control Unit 8A0 959 655 B is the central safety system in various Audi models, such as the A6 4A and RS4 B5. As a Bosch unit with OEM number 0285001037, it continuously monitors the crash sensors and, in the event of a collision, controls the deployment of airbags and seatbelt pretensioners.

The primary task of this control unit is to process signals from the crash sensors distributed throughout the vehicle. Within milliseconds, the electronics determine whether an accident has occurred that requires activation of the restraint systems and then trigger them accordingly.

This is a proven Bosch technology that serves as the heart of the passive safety system, ensuring occupant safety. Therefore, its proper functioning is critical for safety.

Why does the Control Unit 8A0 959 655 B fail?

The most common causes of failure for the Control Unit 8A0 959 655 B are stored crash data following an accident, which locks the system, as well as internal electronic defects. These faults result in the airbag warning light staying on continuously and preventing any further communication. Equivalent part numbers such as 8A0 959 655 A are also affected.

A key reason for failure is the storage of crash data after an airbag deployment. This data permanently locks the control unit to prevent reuse without professional diagnostics. Clearing this data requires specialized equipment and expertise.

Another frequent cause is electronic defects on the circuit board. Faulty components like capacitors or the microcontroller can lead to complete failure. In such cases, the unit becomes unresponsive to diagnostic tools, rendering the airbag system entirely inoperative.

These fault patterns also occur in functionally interchangeable variants, including part numbers 8A0 959 655 A, 4B0 959 655 C, and 4B0 959 655 TB, which share similar technical foundations.

How do you recognize a defective Airbag Control Unit 8A0 959 655 B?

A defect in the Control Unit 8A0 959 655 B manifests through distinct symptoms. The most obvious sign is the airbag warning light constantly illuminated on the instrument cluster. Additionally, during workshop diagnostics, it’s no longer possible to access the control unit via a diagnostic tool to read out error codes.

The primary symptom is the persistently lit airbag warning light. It does not turn off after starting the vehicle, indicating a serious, safety‑relevant system fault.

For workshop diagnostics, another clear indication is the lack of communication. When a technician attempts to connect to the airbag control unit using an OBD diagnostic device, the connection fails. The unit does not respond to requests, making further troubleshooting impossible.

In which vehicles is the Control Unit 8A0 959 655 B installed?

The Airbag Control Unit 8A0 959 655 B was primarily installed in various Audi models, including the Audi A6 of series 4A/C4, the sporty RS4 B5, as well as older models such as the Audi 80, 100, and the coupé. At Airbag24, we repair this control unit for many of these vehicles.

At Airbag24, we repair the Control Unit 8A0 959 655 B and equivalent numbers for the following Audi models:

Audi A6 (series 4A / C4), including Avant versions

Audi S6 (series C4)

Audi RS4 (series B5 / 8D, especially model years 1999–2001)

Audi 80 and Audi 100 (series B3 / B4)

Audi coupé, cabriolet, and S2 (based on series B3 and B4)

How is the Control Unit 8A0 959 655 B professionally repaired?

Professional repair at Airbag24 restores full functionality to the Control Unit 8A0 959 655 B. Following a thorough diagnosis, stored crash data is erased, defective electronic components such as capacitors are replaced, and the software is updated. Afterward, the unit functions like new and comes with 2 years' statutory liability for defects.

After sending the removed control unit to Airbag24, the repair process begins with a comprehensive incoming inspection on specialized test benches. Here, all defects—from stored data to hardware failures—are precisely identified.

In the next step, crash data is completely and securely deleted from the unit’s memory. Then, technicians focus on the hardware: faulty components on the circuit board, such as the microcontroller or capacitors, are carefully desoldered and replaced with new, high‑quality parts.

Finally, the device’s software is updated to ensure full compatibility and functionality. A final functional test verifies that the control unit once again communicates flawlessly with the vehicle and correctly manages all safety functions. Each repair concludes with 2 years' statutory liability for defects.