With a gas dryer the trouble shooting is fairly easy as long as you have a good volt meter with ohms and micro farad capability. A previous repair required the gas valve coil replacement. Based on the age of the unit (17years) I replaced both coils again, the igniter kit, flame sensor and the thermal resistor. Generally, determining which part was bad would lead to replacing only that part. Knowing how these work together, I chose to replace all four parts. After cleaning the unit and replacing the parts, dryer was back in working order. It also heats up faster.