The heater valve(mono) must get electrical SHUT-OFF signal from pushbutton. There is power to the valve any time the key is on, so when you ask for A/C the pushbutton sends GROUND signal to shut the valve off. Water flows thru the valve unless it receives a command to shut off. You should check to see if the valve is getting this power & ground signal. HEAT will always overcome the A/C system.

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