I have a 2010 1.9 diesel estate with a problem heater. The heater starts to blow hot air without me touching the temperature control. The problem used to appear only after about an hour's journey but lately it has started to happen sporadically at any time. Twiddling the temp.control sometimes brings the temp.down, but not always. The engine runs at normal temperature and never overheated. The local dealer cannot pinpoint the problem despite running a computer check. Sometimes I have to shut all the vents and open the Windows because of the heat in the cabin. Can anyone help please?

Sounds to me like one of the sensors has got a fault. I would have thought that a VCDS scan would have identified that though. I had issues with temperature control in my 2010 vRS and it turned out that the near side vent sensor needed replacing.