atwood 7920-II FURNACE ONT IGNITE, fan comes on,no clicking then shuts off,

is it even starting the fan?  thats kind of a little vauge for anyone to try help.  on that furnace when you turn on the thermostat it sends power to the fan blower (via the board), the circuit braker, then the overload switch, then the sail switch and finaly to the main board.  from that point the board controles the gas valves, the ignightor and flame sensor.  

I see way to many people posting their furnace won't start after they changed the sail switch.  it is rarly a actual issue as it is a simmple switch with a fin that gets blowen closed by the fan.  usaly if some one replaces that they did somthing else in conjunction and it happened to fix the problem so now all sw see as soon as you mention furnace issues, is some one saying to change the sail switch.  

a very simple way to determin the intergraty of that who electronic path up to the board is to have some one stand outside by the exhaust vent of the furnace and listen while you turn on the thermostat.  if you fast you can do it yourself.  you should here the fan start up and start blowing air, then while your listining you should hear a click of the gas valves, and shortly after you should hear the ignightor cycling.  if you hear that, the whole sustem to the board is good so you can focus on the board its self, the ignighter, or the flame senser.   or is your combustion chamber filled with bugs..  yes I had that issue when I bought my camper.  took me a while to get everything running good.