Dead Alarm Battery Causes RS4 Security System Chaos

As someone with moderate car repair experience, I dealt with a very similar issue on my RS6 from 2019. The symptoms were nearly identical, alarm system failure and central locking problems despite a healthy main car battery. After multiple error messages, my workshop discovered the auxiliary battery was completely discharged and beyond recovery. These dedicated alarm system batteries typically last 4-5 years before needing replacement. The electrical issue was causing the sporadic behavior you described. The repair involved: Full system diagnosis, Replacement of the auxiliary battery, Recoding of the alarm system, Battery check of the main battery Total cost was 330 Euro and the work took about 2 hours. Since the fix, all alarm and central locking functions have worked perfectly. To provide more specific guidance for your case, could you share: Build year of your RS4, How long youve had these symptoms, When the auxiliary battery was last replaced, Whether you notice any other electrical issues These details would help determine if your situation matches what I experienced and if a similar repair approach would work.

Thanks for the insight. My 2013 RS4 just had its service at 39940KM. The auxiliary battery problem started about 2 weeks ago. Based on service records, the alarm system battery was never replaced since the car was new. The battery signal warnings started appearing on cold mornings. Given the age of the car, it makes sense that the auxiliary unit needs replacement. The car battery problem diagnostics already cost me 80 Euro, so I hope the full system diagnosis and replacement will solve all these unusual errors for good.

The age of your RS4 definitely confirms my initial suspicion. Ten years is well beyond the typical lifespan of an auxiliary battery for the alarm system. The cold morning battery signal warnings are a classic symptom of a failing auxiliary unit. In my case, after replacing both the auxiliary battery and having the car alarm system recoded, all the unusual electrical glitches disappeared completely. The battery problem diagnostics you already paid for should count toward the total repair, as it helped identify the root cause. One tip from my experience: make sure the workshop checks the connection points and wiring harness while they have the system open. My technician found slight corrosion on one connector, which they cleaned during the auxiliary battery replacement at no extra cost. Given your car has never had the auxiliary battery replaced, this maintenance was inevitable. The good news is that once properly fixed, you should be trouble-free for another 4-5 years. The solution may feel expensive upfront, but its far better than dealing with unreliable car alarm and locking functions.

Thanks everyone for the helpful responses. Just wanted to update that I went ahead with the repair today. The system diagnosis confirmed the auxiliary battery was indeed the culprit. Total cost came to 410 Euro including the initial battery check fee. The workshop found the original auxiliary battery had completely failed and also noticed some minor corrosion around the connections just as suggested. They cleaned everything, installed the new battery and recoded the alarm system. The whole job took about 2.5 hours. After the repair, the central locking works perfectly again and the car alarm arms properly. No more unusual error messages either. Given the age of my RS4, this was clearly overdue maintenance. At least now I know to budget for auxiliary battery replacement every 4-5 years as preventive maintenance. For anyone else facing similar alarm system issues in the future, getting a proper system diagnosis despite the initial cost is definitely worth it to pinpoint the exact problem.