Engine management light randomly comes on and goes off

Hi Folks, looking for thoughts and maybe some advise. My 2014 S5 65k miles with full service history and oil change every 5k miles is throwing an engine management light but still running perfectly on all modes. The light sometimes will go out after a day or 2 or can be cleared with a Carista and won't come one for a week or 2, car is used most days but mostly around town. As I mentioned there is no degradation in performance, even the mechanic said it drives really well.

I was thinking maybe the PCV was faulty so had it checked as I have an RAC warranty. However, the mechanic, who is an Audi technician but working for a different garage now, had the car for a day and reported the attached. He said it's about 6 hours work just to get to the PCV and if it's at fault the warranty will cover it but if it's OK I'll need to pay for the stripdown and rebuilt which might be close to 4 figures.

Any advice or thoughts? I can do most minor work on a car, breaks, suspension, steering etc but he tells me this is a nightmare of a job. 

Thanks in advance.