Dead Audi S4 Anti-Theft: Alarm Sensor System Failure

Having some experience with similar security alarm issues, I dealt with this exact problem on my 2018 Audi S3. The symptoms matched perfectly, no response from the anti-theft system and a completely dead alarm indicator. After checking basic things like fuses, my car alarm system needed recoding because the optical locking sequence got corrupted. The battery issue you mentioned could be related, as power fluctuations sometimes cause the alarm module to lose its programming. The fix was relatively straightforward and cost 45 Euro for the diagnostic and recoding procedure. The workshop connected their diagnostic tool to reset and recode the security system parameters. This resolved both the LED indicator and locking response issues. To better assist with your specific case, could you share: Have you noticed any other electrical issues lately? Did this start suddenly or gradually get worse? Does your key fob still work normally for locking/unlocking? Any recent battery replacements or electrical work done? The car alarm symptoms can vary between different Audi models, so these details would help determine if your issue matches what I experienced or if it might be something else.

Last service was at 167000 KM but problem persists even after that. Had another look this morning and noticed my immobilizer warning light flickering out of nowhere, which it never did before. Battery voltage tested fine at 12.6V. Weird thing is the alarm control works fine with the physical key, just not with the remote. Definitely feels more like a module issue than a mechanical one since everything else is working normally. Will need that diagnostic scan to pinpoint the exact cause.

Thanks for those additional details. When my S3 showed similar alarm control issues, I also noticed that flickering immobilizer light. This actually helped point to the root cause, it was the alarm control module losing communication with the car's central electronics. The fact that your physical key works but the remote doesnt suggests the anti-theft system itself is functional, but theres likely a communication breakdown between components. In my case, the module needed both a reset and recoding because the security parameters got corrupted. The battery voltage reading of 12.6V rules out a simple battery issue, which matches what I experienced. These false alarm triggers and system malfunctions are often tied to the module itself rather than power problems. Before going to the workshop, try disconnecting your battery for about 30 minutes, this sometimes resets the alarm control unit. It worked temporarily for mine before needing proper recoding. If that doesnt help, professional diagnostic scanning will likely be needed to properly recode the system. Would you mind checking if your key fob battery is good? While it probably isnt the main issue given the immobilizer light behavior, its worth ruling out.

Thanks for the detailed follow-up. I noticed that flickering immobilizer warning light just this morning, good catch on that being a potential clue. Will definitely try the battery disconnect trick tonight, makes sense to attempt a system reset before paying for diagnostics. Already swapped the key fob battery with a new one last week, so I know thats not the issue. The fact about corruption in the security parameters is interesting, as this started right after that last service. Wonder if some power failure during maintenance could have triggered this. At least its somewhat reassuring to hear the fix was straightforward in your case and didnt require replacing the entire module. Will update once I try the battery disconnect. If that doesnt work, looks like I need that professional diagnostic scan to sort out the alarm control unit. Really hoping its just a matter of recoding like in your case.