Which model furnace is it? The Atwoods have an LED on the circuit board that flashes codes when there's a problem. Here's a code list:
Internal Circuit Board fault Steady on, no flashing
Limit switch/Airflow problems 1 flash with 3-second pause
Flame Sense fault 2 flashes with 3-second pause
Ignition Lockout Fault 3 flashes with 3-second pause
And here's a good troubleshooting guide:
http://www.rvforum.net/miscfiles/Furnace_Trouble-2.pdf
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-Scott, DW, DG, DB, and DD
2011 Passport 2590BH
2009 Ford F150 SuperCrew F
X4 5.4L w/Max Tow