On ours the propane pressure was set too high
I had a similar problem on our furnace in our 2018 Four Winds 24F. The furnace would light for perhaps 20 seconds, then go out. A friend who is a technician found that the gas pressure at the main regulator by the gas tank was set too high. He reduced it to the recommended pressure. The problem persisted, so he removed the furnace and ran it on the bench. He found that the regulator in the furnace was also set too high and the huge flame was overheating the sensors. He reduced it and it has run perfectly for a year now.
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