If your dryer seems to run forever, it could be because of a clogged vent or internal ductwork. Your dryer may have an automatic cycle that turns off the dryer when the clothes are dry. It does this with a special thermostat or moisture-sensing system.
Normally, this is what happens during an automatic cycle:
The thermostat tells the dryer to heat until the interior of the dryer reaches a pre-set temperature--say 135 degrees.
When the dryer reaches the pre-set temperature, the thermostat tells the timer to begin advancing. (If there's a moisture sensor, the timer advances only if the moisture content of the clothing is low enough.)
The timer advances until the interior cools, then the thermostat tells the timer to stop advancing, and tells the dryer to start heating again.
This cycle continues until the clothes are dry. But…if the vent is clogged, the dryer may never reach the proper operating temperature, so it doesn't send the signal to the timer and the dryer continues to run indefinitely, even if the clothes are completely dry. To fix the problem, clean the vent and/or internal ductwork.
Your dryer's heating element may be partially burned out. If it is, your dryer still heats, but at such a low temperature that it takes three or four times as long to dry the clothes. If the element is partially burned out, replace the heating element. You can check for an ohm reading but will need to refer to the service manual for proper reading. This sometimes on the tech sheet located somewhere on the dryer. Usually between 9-13 ohms.
Although this isn't a common problem, one of the thermostats that controls the temperature in your dryer may break and cause the dryer to heat poorly. If so, you need to replace it. The thermostat is usually a small, round, black device mounted to an oblong steel plate. The plate is mounted to the internal ductwork . This thermostat usually has four wires going to it. Check the 2 terminals that are opposite each other and are the closer together of the 2. These 2 terminals should have continuity. If not replace the thermostat.