Mercedes GLS Key Battery Issue

I had a similar issue with my GLS. Key fob stopped working properly and the car was complaining about the electronic key. Have you tried the spare key to see if the problem persists?

The spare key does work, but that confirms it's the other key then, right? Anything else it could be besides the key battery actually being flat?

Yes, based on that, it's almost certainly the key battery. I’d advise getting it checked at a workshop to confirm, but a new battery should resolve the issue.

Yep, you nailed it. Replaced the key battery in my car key, cost me 37€ and everything's back to normal. Thanks!