All dryers have two main thermostats: an operating thermostat that regulates the internal temperature of the dryer and a high-limit safety thermostat that prevents the dryer from overheating. If either thermostat stops working, your dryer will cease to function properly. It will either fail to heat up or quickly overheat and shut down.
You can test whether the dryer's thermostats are working by using an ohmmeter. The ohmmeter will measure the thermostats' continuity and tell you whether they need to be replaced.
Disconnect the dryer from the electrical socket. Slide the dryer away from the wall and disconnect the dryer vent from the dryer's rear panel.
Using the appropriate screwdriver, remove the screws holding the dryer's rear panel in place. Slide the rear panel off to the side of the dryer. Locate the operating thermostat on the dryer's exhaust vent housing. It is typically found near the bottom of the housing.
Unplug the wires from the thermostat by sliding them off the thermostat's contact posts. Grab and pull the wires by the connectors -- not by the wires, themselves.
Set your ohmmeter to the "R X 1" setting. Touch one probe to a contact post on the thermostat. Touch the other probe to the other post. Measure the continuity. If the thermostat is cold, you should get a reading of "zero." If the thermostat has heated up, you should get a reading of "infinity." If you get any other reading, your thermostat is bad and should be replaced.
Repeat the process to test the high-limit thermostat. The high-limit thermostat is attached to the heating element housing. On many dryers, the heating element is in the back of the dryer on the right side. If your dryer does not have the heating element in the back, it will be located behind the drum inside the dryer.
Raise the top panel on your dryer, typically by prying it up with a putty knife. The thermostat will be on the dryer's back wall. Test the thermostat by using the same method as you used on the operating thermostat.
Never try to repair your dryer while it is still plugged in.