The ignitor is an important component of a gas dryer, but even if it's brand new, the dryer may not heat for several reasons. Some problems are related to air flow, because air is necessary for combustion of the gas, and some are related to other components of the dryer that may be faulty. Before embarking on a troubleshooting mission, however, make sure that the gas valve is open and the tank isn't empty.
Operation of a Gas Dryer
When you turn on your gas dryer, it begins to suck air into the combustion chamber at the same time that the ignitor begins to glow. When the ignitor is hot enough, a thermostat opens the gas valve and allows gas into the chamber, where it ignites. The burning gas heats the air, which then flows through the drum and continues out the exhaust port. The circuitry of the dryer includes two thermostats and a fuse that can shut off the gas when the temperature reaches a preset limit. If any of these mechanisms is faulty, the gas valve won't open.
Air Flow Problems
Because the burner needs air to combust the gas, it's important for the air inlet to be unobstructed. The inlet is usually on the back of the dryer, so the dryer should be in an open space several inches inches from the wall. Blockages in the dryer exhaust can also prevent the dryer from heating, but for a different reason. If air can't get out of the combustion chamber, the chamber overheats, and the thermostat shuts off the gas to protect the dryer. If the condition persists, the fuse can blow, and the gas valve won't open until you replace the fuse.
You'll have to unplug the dryer, shut off the gas and remove the cover to test the thermostats and thermal fuse if you suspect one of them to be faulty. You may need to consult the dryer manual in order to locate them. Test continuity with an ohmmeter. When you touch the leads to the terminals of the fuse or the thermostat, a reading of infinite resistance means the component needs to be replaced. You should also test the flame sensor in the same way. It's located in the combustion chamber near the ignitor.
Poor air exhaust creates the high temperatures that damage dryer components, and is often the result of blocked vents or a blocked lint trap. Besides making sure that the vent opening is free from lint, you should clear out the vents once a year with a vent brush or a vacuum. Although it doesn't happen very often, the burner coils can fail, and when this happens, the ignitor may cycle on and off, but a flame never appears. The coils can fail in the middle of the drying cycle, and when this happens, the clothes take too long to dry.
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