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?
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