Airbag control unit with part number 4B0 959 655 C

What is the function of the airbag control unit 4B0 959 655 C?

The airbag control unit with part number 4B0 959 655 C serves as the central control module for the occupant protection system in certain Audi models, particularly the Audi A6 C4 and C5. It processes data from crash sensors and is responsible for triggering airbags and seatbelt pretensioners promptly in the event of a collision.

As the brain of the passive safety system, this module continuously monitors all connected components—from impact sensors located at the front and sides of the vehicle to the ignition circuits of individual airbags. Its proper functioning is critical to ensuring the safety of vehicle occupants.

In the event of a collision, the control unit identified by 4B0959655C analyzes incoming sensor data within milliseconds to assess the severity of the impact. Based on this analysis, it determines which restraint systems need to be activated to provide optimal protection for the occupants.

Why does the control unit 4B0 959 655 C fail?

The most common causes of failure for the control unit 4B0 959 655 C are stored crash data following an accident, which locks the device, as well as general electronic malfunctions due to aging. The latter often involve defective components such as capacitors or issues on the circuit board that lead to internal system errors.

A major cause of failure is the storage of so‑called “crash data.” After an accident in which the airbags have deployed, the control unit writes a permanent fault entry into its memory. This lockout is intentionally designed by the manufacturer to prevent further use of the unit without professional inspection and reset (crash reset).

Even without an accident, age‑related wear can also result in defects. Electronic components on the circuit board—such as capacitors or memory chips—may fail after years of operation, manifesting as an “internal control unit error” despite the vehicle being accident‑free.

The control unit is also compatible with part number 4B0 959 655 G. The fault patterns are identical across both variants, as they share a similar technical foundation. Repair is equally feasible for either version.

How do you recognize a defect in the 4B0 959 655 C?

The clearest symptom of a defect in the control unit 4B0 959 655 C is the permanently illuminated airbag warning light on the instrument cluster. This indicates a serious malfunction in the restraint system, meaning the airbags may not deploy in the event of a crash.

The airbag indicator light does not turn off after starting the vehicle; instead, it remains lit continuously. This is not an intermittent issue but a persistent condition signaling a non‑erasable fault code stored in the control unit.

In the workshop, the defect often becomes apparent when the control unit no longer responds to diagnostic requests or displays a specific error code such as “control unit defective” or “internal error.” Conventional diagnostic tools typically cannot clear these faults because the root cause lies in hardware or memory problems.

In which vehicles is the control unit 4B0 959 655 C installed?

The airbag control unit 4B0 959 655 C was primarily used in Audi vehicles. It was especially prevalent in the Audi A6 series of generations C4 (Type 4A) and C5 (Type 4B), including their sporty S‑models and Allroad variants.

At Airbag24, we repair the control unit with part number 4B0 959 655 C for the following specific Audi models produced roughly between 1994 and the early 2000s:

Audi A6 C4 (Type 4A, 1994–1997)

Audi S6 C4

Audi A6 C5 (Type 4B)

Audi Allroad C5 (Type 4B)

For a successful repair, it is crucial to verify the original part number printed on the control unit. Only then can exact compatibility with the vehicle and its system configuration be ensured.

How is the control unit 4B0 959 655 C professionally repaired?

Professional repair offers a cost‑effective alternative to purchasing a new unit and includes detailed diagnostics, clearing of crash data (crash reset), and replacement of defective electronic components. After submission to Airbag24, the unit undergoes a certified process that concludes with a functional test and 2 years’ statutory liability for defects.

The process begins with the proper removal of the control unit from the vehicle and secure shipping. Upon receipt at Airbag24, the unit first undergoes a comprehensive initial inspection to pinpoint the exact nature of the defect—whether it’s a memory issue or a hardware fault on the circuit board.

If a lockout due to crash data is present, the memory contents are professionally reset. For electronic faults, defective components—such as aging capacitors or memory chips—are replaced with new, high-quality parts, restoring the control unit’s original functionality.

Finally, a comprehensive functional test is performed on a specialized test bench, simulating correct communication and operation of all system components. Only after a successful test is the device cleared for return shipment and ready for reinstallation. Every repair at Airbag24 includes 2 years’ statutory liability for defects.