Citan Diesel Electrical Issue Resolved

I had a similar battery problem with my Citan, though it wasn't the alarm system specifically throwing the error. Is the main car battery in good condition? Sometimes a failing main battery can cause all sorts of weird auxiliary errors. Have you checked its voltage output? It might be worth investigating before you assume it's solely an auxiliary battery problem. Just a thought, but have you tried just clearing the error code and seeing if it returns?

Thank you for your swift response. The main car battery seems fine, starts the engine without hesitation. However, I haven't performed a proper voltage test. Is it possible for the main battery to appear functional yet still cause issues with the auxiliary system? I’ll certainly check the voltage output. Did your symptoms completely resolve after addressing the car battery, or did you still need to replace the auxiliary battery?

Yes, it's entirely possible for the main battery to seem okay for starting but still be creating problems. Modern vehicles are very sensitive to voltage fluctuations. In my case, replacing the car battery resolved the issue. However, since you're getting a specific auxiliary battery error, it may indeed need replacing. Given the potential cost and your description, I'd suggest getting a professional system diagnosis at a workshop to confirm before throwing parts at it.

I followed your advice and had the Mercedes Citan's battery and electrical system thoroughly tested. Turns out, the main car battery was indeed the culprit, despite showing no obvious signs of failure. Replacing the main car battery resolved the "Additional battery alarm system defective" error and restored the alarm system's functionality. You saved me from needlessly replacing the auxiliary battery! Thank you again for your invaluable assistance.