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2020 Montana - furnace would come on & after 5-10 seconds the furnace would go out. The fan would continuously run. After 5-10 seconds the furnace would relight and after 5-10 seconds the furnace would go out again. This process would continue for 10-15 minutes before it would time out. Replaced to many parts to describe.
Finally found a memo that stated at point of installion that the furnace was carried by the propane line as a handle and slightly bent the line. Advised to push the line back and to the right slightly- 1/4 - 1/2 inch. Also read about oil in the propane lines.
Repositioned the fuel line. Disconnected the propane line at the bottom of the trailer where the flex line from the regulator attaches to steel pipe and at the furnace inlet. Blew out the line with compressed air at 100 psi. Did not notice any oil or debris.
Furnace works fantastic now. I have been having this problem for over a year. It used to work only with temps above 35 degrees but in the past 6 months has not worked at all.
Installed a drip line at the regulator to take care of any future oil contamination. Search for rv propane line oil contamination and you might be surprised at your findings.
My furnace always acted as if it was starving for fuel. Any repair I would start with blowing out the lines & then progress from there.