Dead Alarm Battery Causing RS6 Electrical Nightmares

As an owner of a 2011 RS4 with some experience in electrical issues, I went through almost the exact same problem last year. The system diagnosis revealed a failing auxiliary battery that was affecting the entire alarm system. Initially, I thought it might be a more complex electrical issue, but after a proper battery check, the workshop confirmed the auxiliary battery was completely discharged due to ageing. These dedicated alarm system batteries typically last 8-10 years before needing replacement. The fix included a full replacement of the auxiliary battery and a system reset, which resolved all the error codes. Total cost was 330 Euro, including labor and parts. The battery alarm issues disappeared completely after the repair. Before suggesting the same solution for your case, it would help to know: Has the auxiliary battery ever been replaced? Are you getting any specific error codes? Does the main battery test show normal readings? Is the alarm triggering out of nowhere or not working at all? This information would help determine if your situation matches what I experienced with my RS4.

Confirming mileage is 112727KM, last service was in June 2023. Thanks for the detailed insight. Regarding your questions, the auxiliary battery is original and has never been replaced. The main system diagnosis shows error code P156B which points to a battery signal issue. The main battery tests fine with normal voltage readings. The alarm system is completely non-responsive, no sounds, no LED indicators, nothing. Your situation sounds very similar to mine. The age of my auxiliary battery matches the typical lifespan you mentioned, so this could explain the complete system failure. Did your error codes clear immediately after the battery replacement, or did it require additional reset procedures?

Thanks for providing those details. Yes, my situation matches yours almost exactly, especially with the complete non-responsiveness of the car alarm system. In my case, after the auxiliary battery replacement, the shop had to perform a specific reset procedure to clear the error codes. The battery signal took about 2-3 start cycles to normalize completely. This is normal, the system needs a few cycles to recognize and adjust to the new battery parameters. The P156B code is the same one I encountered, which typically indicates issues with the auxiliary power supply. Since your main battery tests normal, this strongly suggests the auxiliary battery is the culprit, just as it was in my case. A few technical notes from my experience: The system diagnosis needs to be run again after battery replacement, New auxiliary battery requires proper voltage matching, Complete alarm system reset is essential after replacement, Error memory needs manual clearing Total resolution time was about 2 hours at the shop. Based on current parts pricing, you should expect a cost around 300-350 Euro for a complete fix, fairly close to what I paid last year. Given the identical symptoms and error code, plus the original battery age, I strongly suspect you are dealing with the same issue I had.

Thanks for the comprehensive information about your experience. This definitely helps confirm my suspicions about the auxiliary battery being the root cause. Your matching error code P156B and similar symptoms give me confidence about proceeding with the battery replacement route. The reset procedure details are particularly helpful, knowing I should expect 2-3 start cycles for normalization will prevent unnecessary worry after the fix. Good to know about the system diagnosis and reset requirements too. Will book the car in next week for the auxiliary battery replacement and full alarm system reset. The 300-350 Euro cost seems reasonable for the complete fix. Really hoping this resolves all the error codes and gets the alarm system functioning again, just like in your case. If anyone else reads this thread in the future with similar battery alarm issues on their vehicle, definitely check the auxiliary battery age as a first step in the system diagnosis.