1. frame sensor, 2. pressure switch, 3. control board, 4. inducing fan motor...
modern furnaces are pretty much all controlled by control boards now, old (25 years+) furnaces are still on relays, but be honest, anytime a furnace goes down, 90% is the control board, a good HVAC guy can bypass the control board and just use all relays.
BTW, don't feel bad, I have never replaced one myself either.
