I appreciate Lou's suggestions. Actually you can do a solid amount of troubleshooting without taking the furnace out. If you ended up buying a multimeter, you can check voltages at the pins on the mother board.
If you get a multimeter, please share the make and model of your furnace. Just take the cover off, and you will be able to see the make and model.
A few people have mentioned the control board. If you have high enough propane flow and voltage is solidly above 10.5 volts with a little load, then it's a 95% chance it's the mother board. Those suggestions might be spot on. Nice thing is they are easily replaced without removal of the furnace, and you really don't need to give someone $100+ to replace it.
You can check the propane pressure with a manometer, but very few people have one. One or two people suggested turning the burners on your stove and see if the flames are nice and high. It's easy to just turn a couple burners on and assume it's fine. If the propane pressure isn't high enough, the gas valves aren't going to work. Frankly, changing the regulator is a ~$30 job and often it will fix the problem if it's a propane based issue.
Have a good one.
Colorado Camper Rental