I just moved in and furnace ignites for about 15 seconds then goes out. I've researched everything I can. Even found the orange wire for the press. Switch had "fallen" off and thought it might have been the whole problem, but still the same thing.
From what I found so far I believe it has to be the flame sensor needs cleaning or replacement, but not sure how to get at it or really even know where it is.
Can anybody help me with this?

The flame sensor is in the burner section and looks like a small rod sticking into the flame,
It will have a wire attached to it. To clean , just remove one screw and take it out and sand to a bare metal and reinstall.

Try https://www.askmehelpdesk.com/heating-air-conditioning/heating-cooling-maintenance-repair-troubleshooting-frequently-asked-questions-58313-2.html#post269151

Which sensor for xe90