furnace won't start
 

My Lenox Pulse furnace won't always come on when the t-stat calls for
heat. The fan kicks on just as if it had warmed up enough for it run,
but no heat. If I turn the on/off switch on the side of the furnace off
and on several times it will fire up and run. I can do the same thing
at the t-stat setting it to off and back to heat a couple of time. It
may or may not work the next time it calls for heat.
Any ideas?