Dryer Heats on High all the time

Yes, the contact in the switch are probably dirty or burned.

Possibly a bad thermostat or a defective temperature switch

It may be as simple as checking your lint filter.
If you use fabric softeners sheets, they tend to clog the lint filter holes. Washing it with soap and water will restore it back to normal and will allow the circulation that the dryer needs to properly dry your clothes.

If the dryer has dual or triple heating elements, one element, or the relay controlling one element, could be open circuit, ( failed). mostly likely is a failed heater element. The elements work together or separately to combine the heat settings. there could be a high and low element, that do the different ranges. it could be that when it is on medium or high, it's really a lower combination of heat, and it may not be easy to observe, but on the low settings, the chosen element is completely open and failed and provides not heat at all. if medium and high are really low and medium, and low and medium-low is really nothing, then 1 element is probably open circuit. ( failed).

while the dryer is running.go to the outside of the house and check the air flow,there should be a large amount air being blown out if there is not check the vent and ducting for lint build up and make sure the hose is not kinked.
when there is restricted air flow all the warm moist air will stay in side the dryer causing the clothes to take a long time to dry. i hope this helps you with your dryer

check the blower fan on motor shaft,then all vent piping then the vent hose to the outside for a restriction or lint plug

Best way to test the elements and stats is to disconnect each one in turn, and with your meter set on the ohms range ,test each one in turn for continuity, you should get F.S.D.(full scale deflection) on each component.
Another way to test the element, is to bypas the stats, and connect the 240v suplly directly to the element, it should get red hot, but do not leave it on for more than a few seconds..
If it had gone down to earth, then theoretically it should trip the circuit breaker.
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Thanks.

A1 Pull the dryer out away from the wall. Unhook the vent from the dryer completely. Do not put anything over the dryer exhaust on the back of the dryer. Do a normal load with the vent unhooked. If it dries better or ok like this then the problem is in the dryer venting. Like a blocked vent pipe or a smashed or restricted vent hose.

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