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good evenin, Are you on currently?
Hello, that model number SP745 you gave is from one of the parts inside the furnace (the ignition controller)... it is not the furnace model number that is on a sticker or metal plate mounted in the same large compartment as the gas valve as a rule.
Tell me a lot about you your technical skills background and so that I can coach you most effectively.... if you need in depth coaching and have the needed skills use the 'attach files' link to the left of your send button to attach a well lit, exceedingly well focused photo of the furnace wiring diagram that is glued to the inside of the blower service door.
We can go from there.
This will be a complex diagnosis and repair, and since it is a gas fired furnace with multiple built in hazards I will look in your local area for a well rated furnace repair outfit or two.. For that I need to know which direction you are from the center of the nearest town or city.
Without some furnace related skills this is nothing you should be trying to fix yourself (the college course on this range of technologies is 2 years full time).