There are several reasons a water heater, fridge, or furnace may fail on Propane.. They all work the same way, only the size of the flame (And a few other things) are diffrerent.
I will give you the ignition sequence for all 3 from the time Ignition is called for (Some stuff happens before that on a furnace)
The gas valve is opened
The Control board sends HIGH VOLTAGE to the spark points
Hopefully the flame ignites
The Thermocouple heats up and starts making 0.480 volts (480mV)
The control board stopps the sparking, Senses the 0.480 volts and continues operating till it gets told by the T-Stat that things are hot enough
Control board is involved in every step of this so it is a candidate.. IF you decide it is the control board, Please consider Dinosaur boards, Skip to end of post as to why.
GAS VALVE: Do you have propane? Is the tank valve open? Has the regulaltor gotten plugged with oil? Is the valve working (Listen for the click, and also measure the 12 volt across the coil)
Points: I think (Without checking the book) the proper spacing is 1/4 inch, My furnace came from the factory with 2x that.. My parent's house (oil fired) Furnace was more like 3 ore 4x the proper spacing.
NOTE: CHECK THE BOOK AS I MAY BE WRONG ON THE SPACING.
Thermocouple. Can get dirty and "insulated" so it does not heat fast enough, it can also fail
The wire connecting it to the control board...Bad connection.
Bad grounds (All over the place)
Now why Dinosaur boards?
When my Furnace failed I priced an OEM board, Most every part you get from Attwood is $250.00 and this was no exception (Some are more expensive, dang few less).
The Dinosaur board, SHIPPING INCLUDED, 108
I took it out of the box and layed the old mark-1 Eyeball on it (Inspected it) I am a certified electronics tech and I was seriously impressed by the quality of the materials and workmanship.. I inspected the other side of the board and I have a theory as to what killed my OEM.. It appears the folks at Dinosaur have the same theory as there is a part not present on the OEM that, from it's nature and location, can only have one function.. To prevent the exact failure I think I had.
I am very impressed by this board.
Home is where I park it!