Audi Q3 Won't Start: Immobilizer System Failure Fix

As someone with reasonable experience working on Audis, I faced a similar issue with my 2014 Audi Q5 2.0 TFSI. The security light and immobilizer problems are fairly common in these models. The symptoms you describe match exactly what I experienced, engine not starting or dying immediately with the security light staying on. The car immobilizer was preventing the engine from running properly due to a key signal issue. In my case, the fix was straightforward. The workshop performed a key programming procedure to sync the remote start system with the car immobilizer. Total cost was 45 Euro for the programming. No physical parts needed replacement. Before suggesting specific solutions, could you share: Has the key been damaged or exposed to water recently? Are you using the original key or a copy? Does this happen with all keys you have? Have you noticed any other electrical issues? This information would help determine if your case requires just programming or a more complex immobilizer bypass solution.

Last service was at 13000 KM about 2 months ago. The issue started right after I changed the key battery. I have tried both keys, the original one and the spare. Neither works properly any longer. Strange thing is the central locking still functions normally, just the anti-theft system seems to be acting up. The security light comes on every single time, even when using the original key that worked fine before the battery change.

This confirms my initial suspicion, the key battery change likely disrupted the key signal synchronization with the anti-theft system. I had the exact same experience with my Q5 when changing batteries in both keys. The central locking working but engine not starting is a telltale sign of an immobilizer communication issue rather than a mechanical problem. The car electronics are receiving the basic unlock signal but not the correct immobilizer code. In my case, and likely yours too, no immobilizer bypass was needed. The workshop simply needed to reprogram both keys to restore proper communication with the car electronics. The security light staying on is the system telling you it doesnt recognize the key signal anymore. Quick tip: If you need to start the car urgently before getting it fixed, try holding the key directly against the steering column while starting, sometimes this allows better signal transmission between the key and immobilizer. The fix should cost around 50 Euro for reprogramming both keys. Much cheaper than a full immobilizer bypass or key replacement. Just make sure to use a workshop with proper diagnostic equipment for key programming.

Thanks for the detailed replies. I'll definitely try the steering column trick as a temporary solution. From what you're describing, it does sound like I'm dealing with the same key signal issue after my battery change. I just called a local workshop that specializes in car electronics and they quoted 55 Euro for reprogramming both keys. That's quite reasonable compared to what I feared might be needed. They can look at it tomorrow morning, so hopefully the security light problems will be resolved soon. They mentioned they do this type of key programming quite often with Audis, which is reassuring. Really appreciate the help in narrowing down that this is likely just a synchronization issue with the car immobilizer rather than something more serious. Will update once it's fixed in case anyone else runs into similar symptoms after changing their key batteries.