up_too_late

I just wanted to post my experience with this in case it helps someone in the future. Today I happened to notice that my driver's side window and mirror switches were inoperative. I tested the passenger front and that worked but neither of the passenger rear switches worked. The power seat and lock switches on the Driver's door all worked. This was ultimately resolved by a combination of charging the main battery and disconnecting both the main and auxiliary battery.

My debugging steps (in order):

• Turning the car off and back on did not resolve.
• Scanning with the iCarsoft MBII gave me a current error code C21500 "Communication with the switch group of control module "door" has a malfunction.
• Charging the main battery with my CTEK for several hours did not resolve. I started here because I suspect my main battery (stamped 04/15) is going as the eco mode hasn't worked in several months. This suspicion is further reinforced from a recent issue with my wife's GLE that had error messages stating that several driver aids were disabled which were resolved by charging the battery (we are not driving much due to Covid, and when we do it is mostly short trips).
• Disconnecting/reconnecting main battery did not resolve. I had come across a post about door control modules and one person said he was able to resolve by disconnecting the battery and I figured that was easy enough to warrant a try.
• DIsconnecting/reconnecting BOTH main battery and auxiliary battery DID resolve.

I've taken 2 trips since and it seems to be holding. I will update if it comes back. Now to convince myself to replace the main battery and not wait until there is a starting issue.