Audi S3 Alarm Dead: Hidden Battery Signal Issue

I had a very similar situation with my 2005 Audi RS4. Based on some experience working with car electrical systems, I can share what worked in my case. The symptoms were nearly identical, the car alarm system stopped functioning while the car battery was still good. After initial troubleshooting, it turned out the backup battery for the alarm system had completely discharged due to age, which is a common battery problem in these models. The workshop did a full diagnostic scan and found that the failing backup battery was sending incorrect battery signals to the main alarm system, causing it to shut down. The fix involved: 1. Testing all alarm system components 2. Replacing the backup battery unit 3. Recoding the alarm system 4. Testing all functions including remote locking Total cost was 330 Euro for parts and labor, and the car alarm has worked flawlessly since. To provide more specific advice for your case, could you share: Have you noticed any other electrical issues? Does your remote key still lock/unlock the doors? When did you last replace the main car battery? Are there any other warning lights on the dashboard?

Thanks for the detailed response. After service at 170846 KM, I found the auxiliary battery was indeed failing, triggering incorrect battery alarm signals. The car alarm system would out of nowhere activate even with a healthy main battery. A battery check revealed the backup unit was only holding 20% charge. The auxiliary battery replacement fixed the inconsistent alarm behavior. The whole repair including diagnostic and installation cost 180 Euro. The electrical issue went away completely after the new battery was properly coded to the system. Just want to add this solution for others who might face similar problems with their car alarm systems.

Thanks for sharing your resolution. Pretty much matches my experience exactly, the auxiliary battery degradation really seems to be a common failure point in these models and causes those unusual car alarm activations. My car was showing almost identical symptoms when the backup battery dropped to a critically low charge state. The battery signal issues completely disappeared after replacement, just like in your case. For anyone else reading: a failing auxiliary battery typically shows these symptoms: Unusual car alarm activation, Total alarm system failure, Error codes related to battery signal problems, Normal main car battery but alarm system issues The repair cost can vary but 180 Euro is about right for parts and labor. The important part is making sure the new auxiliary battery gets properly coded, skipping that step can lead to continued system errors. Good to hear you got it sorted for a reasonable price. These alarm system problems can be really frustrating until you track down the root cause in the backup power supply.

I can confirm this solution worked perfectly. The auxiliary battery was definitely the culprit, causing erratic battery signal readings and making the car alarm system malfunction. My backup battery test showed only 20% charge retention, explaining the inconsistent alarm behavior. The repair was straightforward, 180 Euro for a new auxiliary battery including installation and coding. Since the replacement, the car alarm works reliably and there have been no more unusual activations. The main car battery continues to function normally, proving it was specifically an alarm system power supply issue. For those searching with similar problems: get your auxiliary battery tested if your car alarm starts acting up, even if your main car battery checks out fine. The symptoms of unusual activation or complete alarm failure often point to a failing backup power unit rather than a full system problem. The proper coding after installation is crucial, it ensures the new battery sends correct battery signals to the alarm system. Money well spent to resolve those frustrating unusual alarms and system failures.