Internal heater has stopped working, only blows cold air. This happened a few times before but it would usually start working again.This time though it seems to be staying cold. Engine isn't overheating and both pipes going to the heater matrix get hot so there is a flow of coolant going into the matrix. Hot/cold flap is working, I stripped down the center consul partially and could feel that the heater matrix wasn't warming up. The Astra I have has air con and according to my Haynes manual there is a brief mention of a coolant cut of valve in the heater matrix with two wires going to it but that is the only mention of it in the entire manual. Could someone confirm that there is such a device and that this could stop the flow of coolant into the matrix if faulty.