This guide requires a little knowledge of how heaters works. Basic tools and mechanical knowledge along with standard safety protocols should be used. be very careful when dealing with Propane as it highly combustable. Also ignitor wire caries an extremely high voltage and should be treated with caution. Here is what I told one customer:
I noticed your auction for the circuit board for an older Hydroflame furnace.
Last week I started mine and it came on for about 5 minutes and then shut off ,
I waited about 10 minutes and started it again just fine but 5 minutes later it shut
off again and now it will not do anything. I have checked fuses and checked that the
12 volts is getting to it . Is there any way I can check to find out what needs replacing
Here is what I wrote him
Check to make sure there are no external air leaks into combustion chamber causing your unit to run lean and that inlet and outlet are not blocked modified or obstructed as this will change fuel mixture. If all of these criteria have been met check the flame to see if it is yellow or blue. It should be blue. If it is yellow you have an air intake restriction if blue proceed to pulling out igniter and inspect to make sure the tips are not corroded or melted or possibly bent due to long heat exposure. You can sand the igniter with 2000 grit sand paper and re-insert it. Check to make sure blower speed is adequate and motor is getting at least 12 Volts. If error speed is to slow excessive heat will build up and trigger high limit switch to activate. you may also have a fault high limit switch you can check this by attaching a test light to the output soda of the high limit switch wire which we be the white wire on a hydroflame. If light goes off you know the high limit switch triggered occurrence. Test the input side of the limit switch to verify the light is on. If it is on and you are getting good fan speed meaning a lot of airflow then suspect that the high limit switch is bad and replace it. If possible measure temp with a infrared temp gun to insure it is prematurely activating. Temp is marked on the HL. If the HLS is good and still doesnt go out I suspect the board could possibly be failing. If that is the case next step would be to test the board. Verify that you are getting power to the board when it shuts down. Check to see if board off shuts of gas valve. If there is power but shuts off gas valve suspect the sense probe wire or the board is malfunctioning.
Also, there is also a sail switch that allows voltage to the board after the motor on the blower spins creating wind. This should be an in and out wire on the heater. Sequence is power on, power applied to blower, power passes through sail switch if blower is on, then power passes through HLS If cool enough normally closed, then gas valve is opened. Good luck troubleshooting your heater.
Einstiens RV Parts and Electronics