Mercedes C Klasse Diesel Battery Alarm Problem Solved

I had a similar issue with my old BMW 3 Series. It turned out to be a weak car battery. The IBS flagged low voltage, which disabled some features, including the alarm. Have you done a load test on the battery? A voltage reading alone might not tell you enough. Also, check the quiescent current draw.

Thanks for the quick reply. Can you explain how to check the quiescent current draw and what values would signal a problem? I know about load tests, but I'm less familiar with quiescent current diagnostics.

To check quiescent current, use a multimeter in series with the battery's negative terminal, making sure all systems are 'asleep'. If the current draw is much higher than the manufacturer's spec (usually milliamps), there might be a parasitic drain. Given your symptoms and error codes, I'd still suggest a proper diagnosis from a technician. The repair cost seems fair for a battery and diagnostics.

I appreciate the thorough guidance. I took the car to a shop, and they confirmed the auxiliary battery was bad. Replacing it fixed the alarm issue, just as you thought. Thanks again for the help.