Airbag control unit with part number 4B0 959 655 G

What is the function of Airbag Control Unit 4B0 959 655 G?

The airbag control unit with part number 4B0 959 655 G is the central control unit (SRS ECU) for the occupant protection system in vehicles such as the Audi A6 C5 and S6 C5. It processes signals from crash sensors and is responsible for precisely deploying airbags and seatbelt pretensioners in the event of a collision.

As a core component of the Supplemental Restraint System (SRS), this module continuously monitors the functionality of all connected sensors and ignition circuits. It communicates via the vehicle’s internal data bus (CAN) with other control units to ensure proper operation of the entire safety system.

Why does control unit 4B0 959 655 G fail?

The most common causes of failure for control unit 4B0 959 655 G are internal hardware defects, leading to communication interruptions or the persistent error code 65535. Another cause is the storage of crash data after an accident, which prevents reactivation without a reset.

A typical symptom is complete loss of communication—neither diagnostic tools nor other control units can establish a connection. This is often due to defective components on the circuit board, such as voltage regulators, MOSFETs, or damage to the CAN interface itself.

The internal error 65535 ("control unit defective") is set after the unit’s self-test and cannot be cleared via OBD diagnostics. This error indicates an irreparable internal condition, usually caused by failures in memory chips (EEPROM) or peripheral components of the microcontroller.

After an accident, crash data is permanently stored in the control unit’s memory. This lockout is a safety feature that prevents further use of the device until a professional inspection and a thorough reset of the memory contents have been performed.

It should be noted that there are several hardware variants within this control unit family, including part numbers 4B0 959 655 C and 4B0 959 655 J. Before proceeding with repairs, confirming the exact part number is crucial to ensure compatibility.

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

A defect in control unit 4B0 959 655 G primarily manifests as a constantly illuminated airbag warning light on the instrument cluster. Additionally, diagnostic tools may fail to connect to the control unit, or the specific error code 65535 may appear.

The clearest indication is the persistently lit airbag warning light. It does not turn off after starting the engine, signaling a serious, safety-critical fault stored in the error memory.

Another clear symptom is the inability to perform diagnostics. If a workshop attempts to read the error memory, communication fails—the control unit is no longer accessible via the diagnostic interface.

If the error memory is successfully read, codes such as 65535 or the generic message “control unit defective” directly point to an internal malfunction of the control unit. Such errors generally cannot be resolved simply by clearing them.

In which vehicles is 4B0 959 655 G installed?

The airbag control unit 4B0 959 655 G was mainly used in the Audi C5 platform, including the Audi A6 and the sportier Audi S6 from the late 1990s and early 2000s, both in pre-facelift and facelift versions of these model series.

At Airbag24, we repair the 4B0 959 655 G control unit for the following models:

Audi A6 C5 4B (1997–2001) Audi A6 C5 4B Facelift (2001–2005)

Audi S6 C5 (1999–2001) Audi S6 C5 Facelift (2001–2005)

How is 4B0 959 655 G professionally repaired?

Professional repair at Airbag24 involves an in-depth analysis at the component level, elimination of hardware defects through replacement or reflow of SMD components, resetting stored crash data (EEPROM reset), and a final functional test on a specialized test bench.

Upon receiving the control unit at Airbag24, an initial incoming inspection is conducted to verify the error codes provided by the customer. This is followed by a visual examination of the circuit board for mechanical damage, corrosion, or moisture-related issues.

The electronic repair focuses on hardware: defective SMD components such as voltage regulators, MOSFETs, or ignition circuit parts are identified and replaced professionally. Cold solder joints are re-soldered using the reflow process to restore electrical conductivity.

A key step, especially after an accident, is the crash reset. During this process, crash data stored in the EEPROM is erased, and the control unit’s software is restored to its factory-default state. If necessary, vehicle-specific coding is also reinstated.

Finally, each repaired control unit is tested on a test bench. During this process, all functions—including communication and the activation of ignition circuits—are simulated and verified. We provide 2 years' statutory liability for defects on all repairs performed.