Airbag control unit with part number 420 959 655

What is the function of Airbag Control Unit 420 959 655?

The part number 420 959 655 refers to a central airbag or SRS control unit, primarily installed in the Audi R8 (model series 42). Its main function is to monitor the entire occupant protection system, evaluate crash sensor data, and precisely deploy airbags and seatbelt pretensioners in the event of an accident.

As the “brain” of the passive safety system, this module continuously processes signals from acceleration and pressure sensors distributed throughout the vehicle. It determines within milliseconds whether a collision has occurred and which restraint systems need to be activated.

The proper functioning of this control unit is critical for occupant safety. It ensures that, in an emergency, the correct components—such as front, side, or head airbags, as well as seatbelt pretensioners—are deployed at precisely the right moment.

Why does control unit 420 959 655 fail?

A failure of control unit 420 959 655 is most commonly caused by stored crash data following an accident, which locks the module. Another cause is electronic defects on the circuit board, such as faulty components (e.g., resistors, capacitors) or damaged solder joints, leading to internal malfunctions.

After a collision, the control unit permanently stores so‑called crash data—a manufacturer‑imposed safety measure that necessitates a full system check. The module then becomes locked.

Besides accidents, hardware‑related defects can also occur. These often involve issues on the printed circuit board (board level), including defective SMD resistors, capacitors, problems with the microcontroller’s power supply, or faulty solder connections that result in communication failures or internal malfunctions.

It is important to note that variants such as 420959655Z0K or the similar number 4S0 959 655 exist. These may differ in hardware and software, so always verify the exact marking on the device itself before proceeding with repairs.

How do you recognize a defect in control unit 420 959 655?

A defective airbag control unit 420 959 655 typically manifests as a red airbag or SRS warning light that remains illuminated continuously on the instrument panel. Another clear indication is when the control unit cannot be accessed via a diagnostic tool and no longer allows communication.

The most noticeable symptom is the persistently lit airbag warning light. It does not turn off after starting the vehicle, indicating a critical fault stored in the error memory of the restraint system.

In many cases, attempts to read the error memory fail—the control unit does not respond to requests from the diagnostic tool. This communication breakdown often points to a severe internal hardware defect.

Even when communication is possible, error codes are frequently recorded, indicating either an “internal control unit error” or a “crash event stored.” Such errors cannot be cleared using standard workshop testers.

In which vehicles is control unit 420 959 655 installed?

The airbag control unit with part number 420 959 655, along with its direct variants, is specifically designed for the first-generation Audi R8. It is used in both the original model and the facelifted version, and repairs are available for both generations.

At Airbag24, we professionally and reliably repair control unit 420 959 655 for the following models:

Audi:

R8 (model series 42), years 2006–2012

R8 (model series 42 facelift), years 2012–2015

Since there are different hardware and software versions (e.g., 420959655Z0K), it is essential to carefully read the exact part number from the sticker on the control unit before removal to ensure correct identification and repair.

How does repair work at Airbag24?

Upon receiving your control unit 420 959 655 at Airbag24, we conduct a comprehensive analysis. Depending on the nature of the fault, we perform electronic repairs at the component level. Finally, a functional test is carried out on a test bench before returning the repaired unit to you with 2 years’ statutory liability for defects. With this control unit, erasing the crash data is often not possible. If the crash data needs to be erased, please contact us briefly before sending the unit in – we will check your part number.

The process begins with reading the internal error and crash memory to pinpoint the exact cause. Based on this diagnosis, we select the appropriate repair method.

In case of hardware defects, our technicians open the housing, inspect the circuit board, and expertly replace faulty electronic components such as resistors or capacitors.

After repair, the control unit comes back with all vehicle data and is ready for use immediately after reinstallation — no coding or programming is necessary. The final system test ensures that the control unit communicates flawlessly and performs all functions correctly. We provide 2 years' statutory liability for defects on every repair.