It's the vacuum servo that opens and closes the hot water to the heater core. When no vacuum is applied it's closed. If the electric actuated solenoid still works and you press The climate control buttons for temperature higher than ambient, it should open. You can use a hand vacuum pump to test the diaphragm / valve. I have already replaced this once because the electric solenoid became sticky and wouldn't open even with a vacuum applied by the vehicle. I would check for leaks and vacuum and also that the solenoid works by carefully applying 12V to the plug disconnected. It could be stuck open also.
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