Well, I took the cover off and the LED was blinking, but you can't see
it with the cover on. 3 flashes. According to the manual, that means
"Pressure switch circuit not closed; induced draft blower IS
operating", which it further says could be any of the following things
being wrong:

1) Pressure switch hose is blocked, pinched or connected improperly.
2) Blocked flue or weak induced draft blower.
3) Incorrect pressure switch setpoint or malfunctioning switch
contacts.
4) Loose or improperly connecting wiring.

Well, if the wiring went bad I'll have to call in a repairman, but I
could probably fix the others, if I knew what they were. I don't have
a diagram of the unit labeling each part. Anybody know where I can
find some good diagrams? I'm googling now, but haven't found anything
yet... It's a gas furnace.