Gas dryers sometimes fail to heat up due to clogged vents, blown fuses or other broken parts. In addition, About.com recommends troubleshooting by checking the gas valve where the dryer connects to the supply line. In order for the dryer to heat, this valve must be fully open. The gas valve is off when the handle is perpendicular to the supply line and on when it is parallel to the line.
SF Gate reminds gas dryer owners that these devices require both airflow and heat to dry their clothes. If airflow is restricted by overloading the dryer, a dryer sheet stuck over the exhaust vent or a clogged vent pipe, the heating chamber overheats. Sometimes this causes the thermal fuse to blow, preventing the appliance from opening its gas valves. Located on the exhaust duct, the fuse requires use of an ohm meter to check. If, after disconnecting the thermal fuse, it returns any reading other than zero on the meter when the leads touch the terminals, the fuse should be replaced.
Gas dryers use an electric conductor to supply heat to ignite the gas. When the machine is set to the on position, this conductor glows red like an electric heater. In addition, gas dryers use electronic coils to control gas valves. On older dryers, these sometimes wear out. An ohm meter is also useful in checking these parts. SF Gates recommends replacement of any part that does not return the zero reading.