Normal auto A/C vent temperature

A/C thermal expansion valve.

Higher than normal auto a/c vent temperature.

Heater control valve malfunction

When the A/C is in operation, the duct outlet temperature is higher than normal on a vehicle equipped with a normally closed vacuum heater control valve.

Heater control valve stuck-open.

A stuck-open heater control valve would allow the heater core to add heat to the cooled air reducing the effectiveness of the a/c system.

With a normally closed heater control valve, vacuum would be applied to the valve to cause it to open and add heated air.

Vacuum would not be applied to the valve in A/C mode so a vacuum leak would not cause this problem.

Heater control valve stuck-closed.

A stuck-closed valve would restrict coolant flow to the heater core and the complaint would be no heat.