Audi S4 C4 4A airbag control unit repair

What is an Audi S4 C4 4A airbag control unit?

In an emergency, the Audi S4 C4 4A airbag control unit receives the signals from the crash sensors and decides within milliseconds which airbags and seatbelt pretensioners fire. In addition, it monitors the entire restraint system without interruption and reports faults via the airbag warning light.

When a sensor registers an impact, every millisecond counts. The control unit evaluates the readings and releases the appropriate restraints. During normal driving, by contrast, it continuously checks whether sensors, squib circuits and airbags are ready for use. In the Audi S4 C4 4A (1991 to 1994) it forms the SRS hub, that is, the control unit for the entire restraint system.

If it has registered an accident, it stores crash data and locks itself. This becomes visible in the cockpit: the airbag warning light reports the condition.

Why is the Audi S4 C4 4A airbag control unit defective?

The most common cause is an accident: the crash data stored in the process block the unit, even if no airbag has fired. Other possible causes are aged electronic components, cold solder joints, moisture and corrosion on connectors, and voltage spikes.

An ordinary diagnostic tool cannot remove this crash data. That is why the control unit stays blocked, even if sensors, wiring and airbags work perfectly.

Regardless of an accident, time takes its toll on the electronics: components age, solder joints turn cold, and moisture and corrosion attack connectors. Voltage spikes also do harm, for example with a weak battery or when jump-starting.

You can recognise a defect by these signs: the airbag warning light stays on permanently, the fault memory cannot be cleared, or the diagnostic tool cannot establish contact with the control unit.

Common error codes for Audi S4 C4 4A airbag control unit repair

The codes point to faults in the power supply, in the squib circuits of the airbags or in the control unit itself. Each code therefore describes one part of the restraint system in the Audi S4 C4 4A.

These codes are read out again and again on the Audi S4 C4 4A, each with a short explanation:

• 65535 → Internal control unit fault, control unit defective
• 00532 → Supply voltage too low, sporadic
• 00588 → Driver airbag squib, resistance fault
• 00589 → Passenger airbag firing circuit, resistance too high
• 00590 → Passenger airbag squib, fault
• 01217 → Driver-side side airbag squib, resistance fault
• 01218 → Passenger-side side airbag squib, resistance fault

A diagnostic tool reads out the fault memory. A single code does not yet prove that the control unit is defective, because an airbag module, the wiring or a connector can equally be to blame. Only the inspection shows which cause applies.

What part numbers are available for the Audi S4 C4 4A airbag control unit repair?

You will find the part number on the label of the control unit, and it determines the exact version. Within the model series of the Audi S4 C4 4A, the numbers differ, depending on equipment and year of manufacture.

Note down the number printed on the label of your unit and enter it in the repair order. This allows the right version to be matched unambiguously. Whether a defect is present is shown only by reading out the fault memory and the inspection.

The route to repair: first have the fault memory read out. Then disconnect the battery and remove the control unit. Create a repair order online and send in the unit. At Airbag24, checking, repairing and testing usually take 3–4 working days.

The repaired control unit comes back with all vehicle data and is ready for use as soon as it is refitted — no coding or programming is required.