Check the timer motor for continuity. If there is no continuity, replace the timer.

Make sure the blower wheel is not slipping on the motor shaft. If it does, replace the blower wheel.

If the flame does not light every time when the igniter glows, then most likely the gas valve coils are bad. If they look like the coils on the following picture, then you can order the kit and simply replace them. If they look different with a lot of wires coming out of the coils, then you have to replace the whole gas valve assembly.

If all of the above is good, replace the cycling and hi-limit thermostats (#31 & #10 on the diagram).

- The timer Part number: AP4047639