In a gas dryer, the flame sensor is a safety device that detects the presence of a flame when the burner is in operation. When the dryer is first turned on, the ignitor will heat up and create a spark to ignite the gas. This spark will also heat up the flame sensor, causing its resistance to decrease, which in turn allows the gas valve to open and supply gas to the burner.
Once the flame is established, the flame sensor will continue to monitor the flame and ensure that it remains lit. If the flame is extinguished for any reason, the flame sensor's resistance will increase and the gas valve will close, preventing the flow of gas to the burner.
The ignitor will only turn off once the dryer completes its cycle. The flame sensor will not close the contacts again once the flame is on, it just monitors the flame to ensure it's present.
Once the flame is established and the dryer is in operation, the ignitor will turn off. It is not needed to keep the flame burning as the gas is now flowing and burning continuously. The gas valve will remain open as long as the flame sensor detects the flame.
The dryer's control board or thermostat will turn off the gas valve and the burner when the desired temperature or time has been reached, ending the drying cycle. The flame sensor will continue to monitor the flame and ensure that it remains lit during the entire operation of the dryer. The flame sensor does not get hot enough to close the contacts again, it just monitors the flame to ensure it's present.