Howdy all, 

2009 Audi A3 Quattro:

The other day on my way into work (50km drive) I had the cruise control set at 110kph and somewhere around the 40kmh mark, there was a blip (which I presumed was a misfire) and a few mins later the check engine light came on. On the drive home it was more less the same...no other issues. Scanned when I got home and got code P2181. Cleared it and was on high alert on the drive in the next morning. So this time, turned HVAC on & off, stayed around the 100kmh an hour speed and no engine light. Temp was at 90'c (centre mark) or slightly under (about 7'c ambient temp). I had heat from the vents (thermostat was open). On the way home I churned it up to 110-115kph and about 10 mins in, same misfire feeling blip and engine light on...no other issues. Got home, scanned and it was P2181 again. Cleared the code. Today I ran errands around town for a few hours...heat blasted, temp centre or just below, speeds below 100kph, and no engine light or blip at all.
Could it be my coolant temp sensor? I find it odd that the engine light comes on with the 110kph+ speeds and what's with the blip?
Anyone have a clue or possible diagnostic approach?
F

According to some other posts on VWvortex, it could be the thermostat. 

I'd look at changing that first and see if it resolves the issue.