Bought a 2010 A3 S Line 2.0tdi about 4 weeks ago. The dpf light came on during the drive home and shortly went off again. About 2 weeks later I arrived home from work and some white smoke came from under the bonnet,smelled of exhaust fumes. I took it to an independent Audi specialist who identified the issue to be a clamp on the dpf. Not covered by warranty. They also found a problem with the thermostat so had that replaced and for peace of mind I had the cam belt and water pump done. £728 later and on my next run to work I had the "Turn off engine and check coolant " message on the screen. I only had water with me so 150 ml of water took it back to required level. Yesterday I noticed that when the heater is on HI its really quite hot,turn it down 1 click to 28 and its cool. Also passenger side is warmer. Sorry for the long winded post but wanted to give as much info as possible. Anyone got any ideas? Could it be an air lock or am I looking at more expense that's probably not covered by my 3 month warranty? 

Almost certainly the problem is an air lock created when the thermostat was changed. The coolant system should have been vented by the mechanic who did the work, so I suggest you take it back and get him to finish the job properly as it can be a devil of a job to do yourself.

Thanks buddy,its going back on Monday...just hope they don't try it on and it costs me an arm and a leg again. Spent more on this car in a month than any of my cars I have owned 🤦‍♂️ first Audi I have had...kinda putting me off haha 😄