S4 Immobilizer Issues Causing No-Start Headaches

As someone with some hands-on experience with car electronics, I dealt with a very similar issue on my 2005 Audi S6. The symptoms matched exactly what you describe, the immobilizer fault was preventing the engine from staying running. After checking basic things like battery voltage and starter motor, my workshop diagnosed it as a key programming issue. The immobilizer system was not properly recognizing the key signal. They performed a complete key programming procedure using their diagnostic equipment to reset the anti-theft system communication between the key and the car computer. The fix cost 45 Euro and took about an hour. No parts needed replacement, it was purely a programming issue. Since then, the car has started perfectly every time. To help diagnose your specific case better, could you provide: The exact error codes you retrieved, Whether this started suddenly or developed gradually, If you recently had any work done on the electrical system, Whether you have a spare key to test with The car immobilizer systems can be tricky, but proper diagnosis with the right equipment usually reveals the exact cause. Let me know those details and I can suggest more specific troubleshooting steps based on my experience.

Last service done at 96301 KM. The error codes from my OBD scanner show P1573 which points to the ignition control system. After trying the suggestions, I can confirm its not a battery issue since all other electronics work fine. The key fob works perfectly for locking/unlocking, so Im confused why the car immobilizer would still prevent starting. Could the remote start unit be creating interference with the factory immobilizer? Would a complete reset of the ignition control module help here? Want to avoid replacing parts unnecessarily if its just a programming issue like in your case.

Thanks for providing those details. Your P1573 code and symptoms are nearly identical to what I experienced with my S6. Initially, I also thought the key fob working meant the immobilizer was fine, but the anti-theft system actually uses a separate signal for engine authorization. The remote start unit could definitely be causing interference. In my case, the workshop found that the aftermarket remote start was creating conflicts with the factory immobilizer bypass signals. They had to recalibrate the communication between the remote start module and the factory anti-theft system. Before replacing the ignition control module, I suggest: 1. Disconnecting the remote start unit temporarily to test if the car starts normally 2. Having a professional scan tool check the key signal strength 3. Performing a complete reset of the immobilizer system The complete reset worked for my S6 without needing new parts. The total diagnostic and reset procedure cost about 80 Euro. If these steps dont resolve it, then investigating the ignition control module would be the next logical step. Make sure your workshop has experience with car electronics and immobilizer systems specifically, not all shops have the right diagnostic equipment for these issues.

Thanks for the detailed response. Your experience with the remote start unit causing interference makes a lot of sense. I checked with my mechanic and we will try disconnecting it first to eliminate that as a possible cause. The P1573 code definitely suggests its related to the immobilizer system rather than a mechanical issue. He quoted 95 Euro for a full diagnostic and potential reset of both the immobilizer and ignition control systems. Seems reasonable given the specialized equipment needed for key programming and system resets. Will try that before considering any component replacements. I will update once we determine if the remote start is creating the immobilizer fault. Really hoping its just a programming issue like in your case rather than needing a new ignition control module. The car has been reliable up until now at 96301 KM, so it would be surprising if hardware is already failing.