A gas dryer blows air through a burner assembly to heat the air. The burner assembly consists essentially of a gas valve, ignitor and in dryers that use a glow type of ignitor, a flame sensor. If the flame sensor fails, the ignitor may not go off and the gas flow will not start.
Before testing the flame sensor, unplug the clothes dryer or shut off the power at the fuse box or breaker panel to avoid an electrical shock hazard.
The easiest way to diagnose the flame sensor is to observe the burner operation. Remove the small access panel in front, select a high temperature setting and start the dryer. Watch the burner assembly, shortly after starting the dryer the ignitor should begin to glow. If it glows for several seconds (up to 15 seconds) and then goes out, then the problem is probably the solenoids (coils). If the ignitor glows and stays on, then the problem is usually the flame sensor.
For testing purposes, the sensor should show continuity at room temperature and no continuity when it is heated by the ignitor.