Hey guys!

I ran a few quick searches, but could not find anything on point, so I am going to try to ask you guys directly.

My girlfriend has been driving her 2009 C63 for a number of years now and it has been bulletproof. But, right before her extended warranty expired, we went to the dealer for a comprehensive inspection. Everything checked out and the dealer ran us a list of recommended maintenance. One of the recommendations was to replace the battery in the trunk due to a low voltage reading. But, the car never had/has any trouble starting. A month or two afterwards, the COMAND system started acting weird. The radio would prompt us that it would turn off in an X amount of minutes unless the car was turned on. The buttons on the COMAND no longer work. The navigation and everything else in COMAND does not work. It seems like everything is OK with the exception of the COMAND.

We thought the problems were due to the dealer's low reading on the battery. So, we then went to the local Autozone to replace the battery. But, the same problems persist. We also went back to check the old battery that was replaced and the guys at Autozone told us that the readings on the battery were OK and that it did not need replacing.

In short, the dealer's reading of the battery was low and told us that it needed replacing. All COMAND functions, including the radio, navigation, etc. stopped working. Everything else seems okay (mechanically, headlamps, courtesy interior lights, a/c, etc.). We replaced the battery, but there was no fix. We got a reading on the old battery and it was OK/did not need replacing.

Any idea what is going on, guys? Any help would be greatly appreciated!

Just an FYI, quick tests with low-end equipment at the auto-parts store is not very reliable in terms of knowing if the battery is / was good or not. It actually takes a decent piece of equipment to be able top put a reasonable load on the battery, analyze the voltage / current profile, etc. Age and real-world performance are really your best bet - if the battery is old and you are starting to have problems with it, then replace it. Lead-acid battery reliability has actually improved a lot in the past decade, but even now you're doing well to get 3-4 years out of a basic sealed lead-acid unit.

So, your battery may actually have ben kaput or close to it. Maybe something in the car now needs to be reset with the new battery installed?

Have someone check the voltage near the dash (e.g. cigarette lighter). Car off, it should be around 12.6 volts. Car running, it should be more like 13.8-14.2. If it is lower than this, then there may still be something wrong.

Quick update - we took the car to the dealer, who was going to charge us $3000 for a new COMAND unit. The tech and service advisor were pretty adamant that the entire system required a replacement. But, even after they replaced the COMAND unit, the problems persisted. It turns out that the medial interface control module malfunctioned. Rather than have that replaced for $1500, we asked the tech to by-pass the error code. Everything is now back to normal/functioning normally. (We cannot use our old ipod anymore, but that's not a big deal as we switched over to the aux input.)