rover heaters

hi, think ive got an air lock in the water system, when I change my heaters to cold the temp gauge goes up high, when it is on hot it stays at half way like normal. the temperature knob controls the green cable behind the dash and it runs through the bulk head, but what does it do or attach to at the other side?



its a 2003 rover 25 1.4 16v btw.

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