Astra G Heater not working

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.
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Yeah, there's a coolant cut-off valve on the end of the matrix. Just look for where the pipes go into the matrix, and you'll find it (you'll have to dismantle part of the dash though).
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manual.

ended up stripping the dash, what a nightmare it turned out especially removing the the coolant drain from under the car to get at the matrix. Sure enough there is a cut- off valve which was in the closed position. After hours of work removing the matrix it turned out that the dash mounted air con/heater control unit was faulty and wasn't actuating the motor to open the valve. Anyone who has a similar problem with the internal heater can get a quick fix without going to all the bother I had by looking for the smaller of the two plugs that go into the back of the heating control unit. The plug has five wires, the blue/black and yellow/black are for the valve motor, connecting a twelve volt feed to these wires will open/close the valve depending which way you put the polarity
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