The FAU here started having issues with not lighting last year. The code being thrown by the board was for a limit switch. These aren’t really limit switches, but rather temp switches, but that’s not the point.
Anyways, after investigating, I determined that there are multiple (3-5) separate temp switches wired in series within an input control circuit. If any of them opens, then the unit goes into fault and shuts down/off.
Okay, fine. I start pulling the switches and continuity check them. Nothing conclusive, but I track down a shop that sells HVAC parts to the public & buy a couple of them. There are different styles. Anyways again, get it put back together and it eventually cycles fully & starts blowing hot air.
Fast forward to earlier this season: Same shit. Got to thinking that the fan motor hasn’t started when any of this has been occurring. Pulled the oil-filled capacitor and checked its capacitance. Yup, NFG. Replaced it and all had been good since.
The lack of air flow due to the fan not starting allowed to much heat to build which tripped the temp switch(s).
Maybe one issue is showing itself as another issue?...
ETA: Seems the issue was diagnosed & fixed. Good deal...