Viano key battery issue

Sounds familiar. I had similar issues with my Vito. Have you tried the spare key to see if the problem persists?

Good call - I'll try the spare. But if the spare works, does that definitely mean it's just the key battery in the other key fob? Or could it be something else causing the low voltage?

Yeah, if the spare works fine, it's almost certainly just the key battery in the first key. I had the same low battery warning. Just replace the battery - it's cheap, but if you don't want to fiddle with it yourself, bring it to a workshop.

Spare key worked fine. Changed the key battery in the original key fob, and everything's back to normal. Cost me 37€ for the battery replacement at the workshop. Appreciate the help!