Learn how your appliance is supposed to operate so you can determine if it is malfunctioning.
The gas valve assembly contains a pressure regulator and two gas valves. The following figure shows an older style "K" series gas valve assembly for Whirlpool manufactured dryers but the same design is common on many brands.
Two gas valves are arranged in series within a single housing so that if all solenoid coils are de-energized and one valve is not seated properly and leaking gas, the second valve will not release that gas from the assembly. The first internal valve uses two separate solenoid coils; a 'holding coil' and a 'booster coil'. Both coils must be energized to open the valve but only the holding coil needs to be energized to hold it open once there. The second internal valve uses a single coil to actuate it which is called the 'secondary coil'.
Newer dryer models may usilize the "M" series White Rodgers gas valve (seen below). While a more compact design, it operates in a similar fashion to the older design described above. The solenoid coils on it are also smaller and more condesened somewhat, one containing both the 'booster' and 'holding coil' in a single unit. The other is the lone 'secondary' coil.
When the dryer control demands heat and the operating thermostat agrees the air temperature is too low, the igniter is the first thing activated in the burner assembly. The igniter gets red hot (over 2000 degrees Fahrenheit) and will ignite the gas when it is released. The igniter (shown in the following circuit diagram) has a low resistance path as long as the radiant sensor is closed.
The radiant sensor is actually a thermostat located near the igniter. When the sensor heats up enough to open the circuit internally, current flows through the secondary coil and the igniter but the current is so low that the igniter starts to cool down. All coils are now energized and gas flows past the still hot igniter and bursts into a blue flame.
The flame will radiate enough heat to keep the radiant sensor open. Power is removed from the burner assembly when the operating thermostat opens up. The gas valves close, the flame goes out and the radiant sensor closes.
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