Thermostats in your engine cooling system
Thermostats open and close with changes in engine temperature, and sometimes become stuck. If stuck open, the engine never warms up properly and there is little or no heat from the heater. A stuck-open thermostat can also be a cause of poor fuel economy.
Stuck closed, a failed thermostat can cause engine overheating because the coolant is never circulated to the radiator. A thermostat replacement is usually a pretty straightforward job, but thermostats may be in somewhat inaccessible locations on some cars, adding to the degree of difficulty during replacement.
When having your mobile A/C system professionally serviced, insist on proper repair procedures and quality replacement parts. Insist on recovery and recycling so that refrigerant can be reused and not released into the atmosphere.