dryer is not heating–replaced heating element, thermostat, and fuse. What am I missing? what is is a thermal cut off?
The thermal cut off and high limit thermostat are safety devices that are on the heater housing. They are designed to shut off current to the heater in the event temperature gets too high. They are not resettable and should be replaced in a pair if either one is bad.
Use a meter to test if one is open, no continuity, and if so replace both and that should fix things.