Interesting, thank you for the info! I'll probably bring it up at the dealer service center next time I have it in for something else. I'll reply to this thread with info on warranty coverage.Same problem on our 2016 250d. Start / Stop hasn’t worked for 2 years - lived with it. But now displaying error Auxiliary Battery Malfunction.
I think the Start / Stop functionality is powered by the Auxiliary Battery - which a Capacitor and not a real battery. This component lives under the passenger footwell and Forum Member TooBad has a thread on this Forum documenting how to replace it.
Not sure whether it’s covered under Warranty?
Be very interested to learn how you get on with this.