Hello, I have a Tempstar, direct vent [intake & exhaust] 90% from 1993: mod NUGS100BH04. On Nov 27th, I replaced the fan control box. It worked perfectly until yesterday morning. It kicked on for an hour, for the morning warm up, then it shut down before achieving the morning temp.

Since then I have: replaced the thermostat, switched it to a different breaker, have over-road the safety switch on the side [completely bi-passed now], have tested the two leads on the fan door switch [both hot]. I am mechanically inclined, could wire your house from the ground up, but not good w fine electronics. I have also tested the new thermostat w the fan in "auto" or "on" modes, NOTHING, the fan will not kick on?

Yes, the new thermostat is set for heat at 70 degrees & my house temp is now 50 degrees. Usually, when standing by the furnace, when I flip the side safety switch to on, the furnace clicks internally. But now, NOTHING. Please I need some help !

Have you checked for good fuse in control board? First step of a furnace the inducer motor should come on (if it doesn`t nothing else will work)
Try spinning it manualy,is it free spinning? If so get the multi meter out ,see if there power going to inducer,if yes,you probably have a bad inducer motor, if no,trace the wires from inducer back to origin,check for power at the contact of wires,

,, let us know what you find