2000 Chrysler Cirrus Not cooling

A thermostat stuck open will not allow the engine to come up to normal temperature. Your gauge will read lower than normal and there will be very low temperature air from the heater.

A thermostat stuck closed will cause engine overheating. The electric fan will be commanded on more than normal and the gauge will read high. Since the AC condenser sits in front of the radiator, air cools it first, so hotter than normal air coming off he radiator should have very little effect. Sometimes, however, the AC refrigerant can't condense back to a liquid efficiently in higher ambient air temps, just like it isn't as efficient in hotter weather.

Stuck thermostats are not very common. At nine years old, it's a lot more likely the system is low on charge.

Caradiodoc