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.
The dryer coils open the gas valve on the dryer. When they recieve full voltage from the radiant heat sensor they open the gas valve the full amount. If your dryer does ignite but still runs on forever the coils are probally failing. They tend to fail as they heat up so they might work hit or miss or if you shut it off and try again later it might work for a few minutes. The dryer coils are inexpensive and easy to replace.
Thermal Fuse / Thermal Cutout / Hi Limit Thermostat
These are all different types of safety fuses located on your gas dryer. These are located in several different areas of the heating duct depending on the model. They are all easy to replace and all you need to do is check them for continuity. If they do not have continuity replace them, if they do they are good. Usually if you get some heat from the dryer these will not be the problem.
Radiant Heat Sensor
This is not the problem if your dryer lights at all. If your dryer is not igniting at all glows but does not light then it could be the radiant heat sensor. This is inexpensive and easy to replace. Just check it for continuity if it does not have any replace it.
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.