I have furnace that when it comes on everything works fine the purge motor comes on the igniter heats up the gas valve comes on and the burnere ignite aand everything looks good until the igniter turns off then the burner shuts down i havent looked at the fault codes on the board what could the problem be none of the limits are tripping out it dose have a flame sensor.Also I converted the gas valve from natural gas to propane before all this started i check the gas pressure and adjusted it to the specs.all help would be appreciated thanks.