Thermal fuses open when they sense heat build up. Usually happens when the dryer is not moving air. I always take a flashlight and look behind the dryer before I move it to check the vent between the dryer and the wall. The flexible vent is easy to pinch when the dryer is pushed back. Also the lint filter only traps part of the lint and so a good portion of it goes into the vent along with the air. Dryer vents that have any length or rise or multiple bends need to be cleaned out once in a while.