If your dryer never quits on its own, then your main selection switch may be bad. Even if the clothes don't dry, the timer should still only run to the end of the cycle and then stop. The problem with the lack of heat could also be caused by a bad timer switch, but that is rare. Usually it is caused by a bad igniter/heat sensor unit. Sometimes you can use a piece of sandpaper and clean the carbon off of the sensor and then it will sense heat when the pilot flame starts, then it will let the gas valve send gas to the main burner. The heat sensor it the part that is in the flame of the pilot light.