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A bad dryer thermostat can fail in such a way that the heat is "stuck" on and won't turn off no matter what setting you put on the dryer. This is a dangerous condition as a dryer can overheat and create a fire hazard. You will need replacement dryer parts, namely a replacement thermostat. Use the same brand that corresponds to your dryer, whether you need Kenmore dryer parts, GE dryer parts, Maytag dryer parts, and etc. Dryers have more than one thermostat, but fortunately many models have them grouped together. These dryer parts are often located in the vent line, under your lint trap or on the housing of the blower. If you don't feel like testing them all with an ohmmeter, there is nothing wrong with replacing them all and see what happens.
You forgot to mention that this problem can also be caused by a plugged exhaust vent or a faulty blower wheel. The evident symptom being that the dryer is getting too hot, yet the clothes are taking a long time to dry.
Perhaps a more logical solution is to not try to do things we aren't qualified to do. Dryers get really hot, much hotter than our hands can usually withstand. How hot a dryer should get is a question for a professional. A simple phone call for information to a trusted local professional will result in much more reliable information than any website can offer.
belt for the dryer installation