Air Flow Problem
If the dryer won't stop, check the venting. A clogged or partially clogged vent will reduce the air flow through the dryer and greatly increase the drying time.
If the dryer is slow, the blower wheel might be broken or plugged. The blower wheel is attached to the drive motor and draws the air through the dryer drum. Remove the vent from the back of the dryer and see if the air flow is strong. If not, check the blower wheel.
Electric dryers have a heating element which warms the air as it passes over. If the dryer is slow, this element may be burned out. The element can be checked for continuity. If it is burned out, the element must be replaced, it is not repairable.
Heating Element Assembly
If the dryer is slow, the heating element assembly may need to be replaced. The heating element assembly warms the air as it passes over. If the heating element is burned out, or any other part of the assembly is defective, it may not heat. If the dryer is slow, and the vent is clear, this is a common next item to test.
Although not common, if the dryer is slow check the lint filter. The lint filter can get clogged with fabric softener and not allow enough air to pass through. Clean the filter of any residue that might prevent proper air flow.
The moisture sensor sends a signal back to the control board as it senses moisture in the clothing. If the dryer is slow, this sensor may not be working properly and may be inaccurately sending signals back to the main circuit board.
High Limit Thermostat
All dryers have a high-limit thermostat to help prevent fires and damage to the dryer. If the dryer is slow, it can be caused by a defective high limit thermostat turning off the burner prematurely. This is not common.
All dryers have a thermostat to regulate the temperature of the air. If the dryer is slow it can be caused by a defective cycling thermostat. This is not common. Check the venting first before replacing a cycling thermostat. A cycling thermostat has normally closed contacts that open on temperature rise. Some thermostats have a bias or "cheater heater" built into the thermostat to add heat so the thermostat will cycle sooner to get the lower temperatures. This heater if applicable will be the smaller terminals in the center. The thermostat and heater can be checked with a meter.
The thermistor senses the temperature of the air in the dryer and cycles the heat on and off. If the dryer won't stop, the thermistor may be defective. This is not common.