Just to say same thing as Richard in a slighty different way ...
A Furnance has a fused 12 volt feed and a good 12 volt return (ground) feed.
B Thermostat sends 12 volts to the furnace on another wire to tell furnace to operate
C If A and B get satisfied then you should hear the furnace blower start to run.
D If A, B, and C then the airflow should close a switch called a sail switch that is airflow activated.
E1 If A, B, C and D then the control board will apply 12 volts to a propane valve and open propane flow to the furnace burner. This will make a "click" sound.
E2 If A, B, C and D then the control board will generate high voltage to a spark gap element next to the burner inside the furnace, and ignite the propane being fed to the burner
F There is a flame sensor next to or part of the spark gap element if the control board senses flame then all is good. If within a couple of seconds the flame sense does not happen then the control board turns off the propane valve.
G If the no flame sensed - turn off propane valve happens then a few seconds will pass to let the propane dissapate and it will start again at E. It will try this 3 times and if no signal on flame sensor on the third try the control board will stop trying and "lock out" the furnace from further tries. Lock out is generally 1 or more hours, you can clear the lock out when troubleshooting by disconnecting the 12 volt power from the furnace for a few seconds and then reconnecting.
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As Richard said, checking connections requires no parts, do that first. You can remove the outside cover and clean, that requires no parts as well. Getting to the actual combustion chamber where the spark gap and flame sensor is requires some substantial dissassembly and in some furnaces the furnace has to be removed to do this. You can get manuals and diagrams online for your particular furnace if you feel like you have the skills to DIY it. Insects can nest inside the burner side of the furnace that has the outside intake and exhaust vents and this can prevent sparking or propane ignition or even keep the whole blower from turning.
Wasps, etc. are attracted to propane, there are screens that mount over the intake and exhaust vents of the furnace that can minimize the risk of insects entering the furnace.
Good luck
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Randy - Manhattan, Kansas
2015 Vista 27N
2020 Ford Escape Hybrid
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