Hi, 

I have a 2020 Q7 Tdi50, which I have owned for 3+ years during which time the MHEV has worked flawlessly. However I recently parked up for a couple of weeks while on holiday and on driving 200 miles home and for the following 10 days the MHEV failed to work.

I have an Audi towbar fitted and yesterday connected my trailer for a couple of journeys (which I know normally automatically cuts out the Stop/start function, in that it is obviously not helpful for an engine to cut out going down hill with 3 tonnes of trailer on the back.)

This morning on driving the car the MHEV was back to working normally.

My conclusion therefore was that plugging in the trailer appears to have reset the Stop/Start and might be useful to anyone else if by chance they have MHEV and a towbar.

Very interesting. I wonder if the battery was low and connecting the trailer encouraged the car to increase power which is what caused it to reset itself? 

Just a theory.