Over the last couple of weeks I've had the catalytic converter warning light glow in my 2012 CC. I had somewhat assumed it was related to my over 5 year old battery which I had truly run into the ground (I could hear the engine struggle to start up). I finally had the battery replaced with the exact same Varta OEM model battery (which sat on my shelf for 8 months, before use). The mechanic ran the computer tests before and after the battery replacement. Beforehand there were a range of faults, but afterwards the faults cleared up.

In my first proper drive after replacing the battery, everything seemed fine (strong engine startup, though start/stop wasn't engaging). After about 15 minutes, start/stop tried engaging, then the battery warning light lit up, together with the catalytic converter warning light, and the engine management system warning light. The car would start up again fine, though with the catalytic converter warning light on. Whenever the car tries to engage start/stop, it fails, and the engine management system warning light and battery warning light comes up. If I disengage the start/stop system, the car drives fine (though the catalytic converter warning remains).

I'm thinking that perhaps the battery has not been sufficiently charged through driving (which driving over time may help) but I'm worried that there may be related damage to systems already. I'm going to have the car checked out by the mechanic again tomorrow, but if anyone has any recommendations/advice, I'd be most appreciative.