My thoughts also on the relay.
As I said a quick test is to jumper the 12v wires on the relay feeding the motor.
Which will put 12v direct to the motor.
How it works, 12v into furnace(2 wires) 2 wires come out of furnace to thermostat. One is low voltage other is return to furnace relay.
As Steve said relay is normally closed. The low voltage heats coil in relay, which which closes to complete circuit. Allowing relay to close and sends power to motor and sail switch.
Getting the motor to come on is the first step. Until this happens in that order it’s not possible to get running.
First step check for 12v power to furnace
-power to thermostat which you said you have. Which means you have power to furnace.
- turn on thermostat, which sends power to timed relay. If relay is working, in about 15 to 18 seconds the motor will come on.
This has to happen before anything else.
If this doesn’t happens, then you start testing from there.
Which will be a problem with
No power to furnace, no power to thermostat ( bad board) no signal to relay. Bad thermostat or wire.
Power to relay no fan control. (Bad relay.)
Power to motor, no start (bad motor)
So, once you verify that motor is coming on in that order. You can go on to finding the problem as to why it didn’t fire.
Which will be,
Bad sail switch (possible )
Possible thermistor switch, not likely
Power control board, (possible if no spark or power to thermostat )
No spark control bd, possible electroid, clean adjust, test for continuity.
Bad gas valve, (not likely)
Bees nest in flue causes it to shut down
No gas, air in lines, dirty propane
, dirty burner etc.
You can do this, and you will learn. If your like me and have more time than money. You will figure it out.
I had pm’d you my phone number, if you need to talk through it.
I see Steve jumped in to help, you can reach out to him also. He is sharp and a electrician, I think.
You can’t guess, think or assume, anything. You have to verify each step before you move onto the next.
Myself, I’d remove the unit and work on it on the Bench. And you may have to also, if you haven’t already.