The 6 most common part(s) or condition(s) which cause the symptom Dryer won't stop are listed below starting with the most likely. Check or test each item and watch any available videos.
The most common problem when a dryer is slow is when the vent is clogged. If the dryer is slow, check the vent from the back of the dryer to the outside of the house. If that is clogged, clear it completely and test the dryer. If the dryer is slow after the vent is cleaned, clean the internal ductwork of the dryer. This will require some disassembly.
RepairClinic Item # 1245880
The blower wheel draws air through the dryer and pushes the air out the vent. If the dryer won't stop, the blower wheel may be plugged up or not be spinning properly, which reduces the air flow. Check to see if the blower wheel is securely fastened to the drive motor shaft.
RepairClinic Item # 3479
If the dryer won't stop, the gas valve solenoid coils might be defective. Over time these coils will fail, causing the burner to run intermittently. If the dryer is not heating properly it may not stop. Replace the coils as a set.
RepairClinic Item # 1067810
Dryer sheets and fabric softener can leave a residue on the lint filter, reducing the air flow. If the dryer won't stop, check to be sure air can flow freely, clean the filter as needed.
RepairClinic Item # 481329
If the dryer won't stop, check the high limit thermostat. If defective, the high limit thermostat can turn off the heat prematurely, lengthening the time it takes to dry a full load. This is not common.
RepairClinic Item # 484291
The cycling thermostat regulates the temperature of the air in the dryer. If the dryer won't stop, it may be caused by a defective cycling thermostat turning off the heat too early. This is not common.