Here is a
service manual that includes the AFM series, so you have to pull out the info that pertains to your furnace. The best I could offer from reading some of it is...
- check gas pressure
- chk thermal switch
- chk gas valve operation
- chk sail switch operation
You need to use a multimeter for some of these checks, also the ignition probe is the flame sensor, dunno how to test it.
Gook luck!