Trouble-shooting:
Furnace won’t start
1. It's fairly common for the thermostat's contacts to oxidize, especially between heating seasons when it's not in use. It's easy to diagnose this.
If the furnace won't start even with the thermostat turned all the way up, take off the plastic cover. You'll see a red wire and a white wire going to two screws on the left side of the case.
Using a straightened-out paperclip or a short piece of wire, bridge those two screw terminals together. The furnace should start immediately. If it does, you know you have a thermostat problem. On the other hand, if the furnace *doesn't* start when you bridge the thermostat's screw terminals, then you have a furnace problem.
Usually, moving the lever back and forth a dozen times or so will clean off the contacts and cure the problem. Another method for cleaning the contacts is put a piece of paper (not sandpaper) between the contacts and move it through a couple of times. That should clean the contacts and renew the connection.
Furnace starts, but only blows cold air
1. You're out of propane.
2. The propane isn't igniting, due either to a failed board or crud/spiderwebs in the burner area.
3. The “sail switch” is mal-functioning.
In the furnace's air duct there is a "sail switch"--a movable paddle--that controls the burner's gas valve and ignition circuit. If the burner were allowed to run when the blower wasn't moving air, the furnace would soon overheat, and could cause a fire. The sail switch prevents this by only turning on the gas after its paddle has been blown aside by moving air.
But if your house batteries are low, so the blower isn't blowing fast enough, the sail switch won't be sufficiently deflected by the airflow, and the burner *will never light.
This can also happen if the sail switch is stuck due to rust or cobwebs.
It's a good idea to do what you can to prevent "creatures" from getting into the furnace system in the first place. Many people install simple little screens over the two furnace vents on the outside of the rig. These can be bought at nearly any RV parts store and simply hook on.
You could probably devise something yourself that also do the job.
FurnaceWon't Shut Off:
That being said, the most likely problem would seem to be the card not functioning properly. By removing the thermostat the signal is gone so the card should move to a shut down condition. If you see both burner on (heat) and blower operating, then the card is the most likely problem. A very remote possibility the thermostat wires are rubbed together and shorted together maintaining the call for heat - unlikely in a LD since the wiring is typically very well protected.
The other possible issue might be with the wiring if someone has worked on the furnace anytime recently and created the short between the thermostat wires.
First, pull the thermostat wires off at the board and check for continuity.
If wires check out OK, then remove the card and find someone with a Fenwal tester and verify the operation of the card.
Revised: 25 Mar 11