Audi S8 Alarm System Failure Linked to Immobilizer

As someone with some repair experience, I had a similar issue with my 2004 Audi A8 4.2L. The alarm system and immobilizer problems turned out to be related to faulty alarm control coding. The security alarm would either not activate at all or trigger false alarms out of nowhere. After checking with a workshop, they found the alarm system was incorrectly coded for the optical locking confirmation through the indicators. The fix was relatively straightforward, they needed to recode the alarm control module. Total cost was 45 Euro for diagnostics and recoding. This is a known issue with these models where the immobilizer and alarm system can get out of sync with the central locking. The good news is its usually just a programming issue rather than hardware failure. To help diagnose your specific case, could you share: Are there any other electrical issues? Does the central locking still work normally? When did the problem first start? What error code is showing exactly? This would help determine if your issue matches what I experienced or if its something different.

Thank you for the detailed explanation. I have noticed other electrical symptoms lately. The central locking works intermittently, and the power failure seems connected to a battery issue. The problem started about 2 weeks ago after my last service at 45868 KM. The alarm sensor shows error code P0513, which according to my scan tool relates to the immobilizer system. In addition to the security alarm not working, the car sometimes has trouble starting in cold weather. I will probably need to get this checked professionally since it seems more complex than just a coding issue.

Thanks for providing those additional details. Your symptoms actually sound quite different from my initial diagnosis on the A8. With error code P0513, intermittent central locking, and cold start issues, this points more toward a power supply problem rather than just alarm control coding. When my A8 had similar problems last winter, it turned out the false alarm triggers and power failure issues were caused by a failing battery that wasnt providing consistent voltage to the alarm system. The immobilizer errors were just a symptom of the unstable power supply. The repair involved: Battery replacement (280 Euro), System voltage check (45 Euro), Reset of the alarm control module (35 Euro) After fixing the power supply, all the symptoms disappeared, no more unusual false alarms, the immobilizer worked properly, and cold starts were normal again. Given the mileage and age of your S8, Id suggest having both the battery and charging system tested before diving into complex alarm system repairs. Voltage issues often cause these exact symptoms and error codes in these models.

Thanks for that assessment. It does make a lot of sense that my issue could be power-related rather than directly with the security alarm components. The symptoms match exactly what you described, especially the connection between the battery problems and false alarm behavior. Since the car alarm started acting up right after the last service, I will take it back to have them check the battery and charging system first. The 360 Euro total repair cost you mentioned sounds much better than what I was worried about paying for a complete alarm system replacement. I will have them run a full diagnostic on the power supply and immobilizer system. The cold start issues and error code P0513 really point toward the battery being the root cause rather than a more complex alarm sensor problem. This has been really helpful in narrowing down where to start with the repairs. Will update once I get the battery and charging system tested.