Had the battery replaced at the dealer on Saturday.

Then today, my wife tells me she is getting an Auxiliary Battery Malfunction error.

Anyone have any ideas here? Is that an easy swap on a 2010?

Pretty easy , im sure there are videos on YouTube as im ive used it to change mine out.

why did u change the main battery?? Maybe it was auxiliary battery all this time causing malfunctions

Car was slow starting, then didn't start at all.

i dont know if yours is a 550 but 10-11 did have a starter issues , well not only 550s but mostly.
if you start noticing slower crank withing couple months again u might wanna check that.
but yes , you can get an Aux battery on ebay/amazon under 20 bucks. And install it yourself within 30 min tops.

Easy swap and cheap like belarus27 said. Check out this thread for details - https://mbworld.org/forums/e-class-w...acement-3.html