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The sail switch should have continuity through it when it is closed, look at the diagram I sent, you will see a white wire going into the circuit board, if the sail switch and high limit are good that wire should have voltage when the blower motor starts up, that is the trigger to tell the board to start sparking and open the gas valve. IF you have voltage on that wire then I would have the board tested, most RV shops have a special board tester and can tell if it is bad or not. It will be a process of elimination, look at the manual they do a good job explaining each component.... Robert