I could help more if I had some info on your furnace to see what components it has.
Do you have a wiring diagram ?
At the draft motor, are there any small pencil size tubes connected to it and going to burner area and one to a small round disc type switch with two wires ?
The 2 wires, if there is a pressure switch, one shows 24vac to ground, one shows 0vac to ground with call for heat.
Across them, you read 0vac IF it closed. 24vac IF it stayed open.
If so, could be the tubes are clogged, torn, switch defective.
If not, centrifugal switch in draft motor could be bad.
Draft motor Air Flue piping could be loose or clogged.
Gas Valve will show 24vac at terminals after about 2-3 minutes max time.
IF the GV shows the 24vac, then need to get the gas pressure checked.
That's about all w/o any knowledge on the unit you have.
Hope it has helped some.
Thanks for inquiring with us.
Take care and best wishes,
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