it sounds like a temp sensor, i take it you mean inside the coolant flange on the front of the head? if so it could be someone in the past needed to block the 2nd hole up on a new flange, and all they could find was another temperature sensor to do the job. mk3 gti should have just 1 4 pin temp sensor, the other hole if there is one is blanked. on the side flange sometimes there is a sensor, this is for the aircon
