Airbag control unit with part number 4B0959655M

What is the function of the airbag control unit 4B0959655M?

The airbag control unit with part number 4B0959655M is the central control module for the occupant protection system (SRS) in Audi C5-platform vehicles. It processes data from crash sensors and is responsible for precisely and reliably deploying airbags and seatbelt pretensioners in the event of a collision, thereby protecting occupants.

As an SRS module (Supplemental Restraint System), it continuously monitors all connected components of the restraint system, including front and side airbags, seatbelt pretensioners, as well as the associated crash-detection sensors and their wiring harnesses.

This unit is specifically designed for the electronic architecture of models such as the Audi A6, S6, and RS6 from the C5 series and communicates with other vehicle control units to ensure flawless operation of the entire safety system.

Why does the control unit 4B0959655M fail?

The most common cause of failure for the control unit 4B0959655M is the storage of crash data (crash flag) after a collision, which prevents further functionality. Electronic defects on the circuit board—such as failed capacitors, corroded solder joints, or faulty memory chips (EEPROM/Flash)—are also typical sources of failure.

Following a collision, the unit stores irreversible crash data and locks itself for safety reasons. Even if the physical unit remains intact, this software-based lock prevents further use and results in the airbag warning light remaining constantly illuminated.

Purely electronic component-level failures are also widespread. These include malfunctioning voltage regulators, defective capacitors, or damaged SMD components on the board. Corroded contacts at the connector pins can likewise disrupt communication and lead to complete system failure.

It’s important to note that there are several hardware variants of this control unit. Related part numbers such as 4B0959655AA, 4B0959655G, 4B0959655C, or 4B0959655P are often affected as well, sharing a similar technical foundation.

How do you recognize a defect in the 4B0959655M?

A defect in the airbag control unit 4B0959655M primarily manifests as a permanently illuminated airbag warning light on the instrument cluster. Often, the control unit can no longer be accessed by diagnostic equipment (no communication), or specific fault codes are stored in the error memory.

The most obvious symptom is the persistently lit SRS or airbag warning light, which fails to extinguish after starting the vehicle—indicating a serious, safety-critical system error.

Another clear sign occurs when a professional workshop attempts to read the error memory but cannot establish communication with the airbag control unit. The unit does not respond to diagnostic requests and becomes unreachable within the system.

In cases where communication is still possible, fault codes are frequently recorded, pointing to either an internal defect in the control unit or stored crash data. This confirms that the root cause lies directly within the control module.

In which vehicles is the control unit 4B0959655M installed?

The airbag control unit 4B0959655M was specifically developed for the Audi C5 platform. It is mainly found across various versions of the Audi A6, S6, and RS6 from this model series. At Airbag24, we have extensive experience repairing this control unit for the entire C5-generation lineup.

At Airbag24, we repair the control unit with part number 4B0959655M for the following Audi models and series:

A6 C5 4B (model years 1997–2001) and A6 C5 4B facelift (model years 2001–2005)

S6 C5 (model years 1999–2001) and S6 C5 facelift (model years 2001–2005)

RS6 C5 (model years 2002–2004)

How is the 4B0959655M professionally repaired?

At Airbag24, repairs on the 4B0959655M follow a rigorous, multi-step process. It begins with electronic diagnostics, followed by addressing the root cause—whether clearing crash data or replacing defective components. Final functional tests on a test bench ensure full operational capability.

After being sent to Airbag24, each control unit undergoes an initial inspection and diagnostic phase. During this stage, the error memory is read, crash data is identified, and the hardware is examined for visible damage such as corrosion or burn marks.

The core of the repair involves work on the circuit board. Depending on the nature of the fault, defective components like capacitors or voltage regulators are replaced, cold solder joints are re-soldered, and the EEPROM or Flash memory is read to safely erase crash data and restore the original configuration.

Subsequently, the control unit is tested on a specialized test bench to verify proper communication and all safety functions. For some units, adaptation to the vehicle configuration (VIN, airbag equipment) is required. We provide 2 years' statutory liability for defects on all repairs.