Suburban and Atwood (Dometic) furnace board fault codes
It is ONE series circuit, not a bunch of switches each wired to the board. 12V comes from the fused coach feed, through the thermostat and the furnace ON/OFF switch (or breaker), and the thermostat output is the board's call-for-heat input. The board turns the blower on. The circuit to the gas valve then runs through the SAIL switch and the LIMIT switch in series before it reaches the valve side of the board. Any one switch open = no gas.
RV furnace: 1 flash (3 s pause)
Air flow / limit fault. Check blower speed, return air, ducts, sail switch and limit switch. Measure 12V both sides of each.
RV furnace: 2 flashes (3 s pause)
Flame sense fault. Electrode position, gap, porcelain, ground.
RV furnace: 3 flashes (3 s pause)
Ignition lockout. Gas supply/pressure, spark, valve. Reset by cycling the thermostat or power.
RV furnace: Steady on
Internal control failure. Verify voltage and inputs, then replace the board.
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Guided diagnosis
Full field guide: Suburban and Atwood (Dometic) furnace board
Codes come from the maker manuals and public service sheets. Your model's own chart always wins, and LP and 120 volt work belongs to a qualified technician.