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When it doesn't fire up does the furnace respond at all or does nothing happen?
That doesn't exactly answer my question. It sounds like you're saying when it doesn't work you have a call for heat but the furnace doesn't respond, nothng happens. If that's the case then the most likely culprit is a problem with the thermostat or the thermostat wiring between the stat and the furnace. A partial break in the wiring or a failing thermostat can cause this
So the furnace responds to a call for heat but just doesn't fire up intermittently? If so you can you tell me what the LED sequence is on the board is when it doesn't fire?
Then you still could have a wiring problem between the stat and the furnace. Did the tech check it out when it wasn't working? If not then his aseertion that's it's all OK is meaningless. That's the nature of intermittent problems and 18 gauge wire is notorious for breaking partially, un-noticed. The other possibilities are much more expensive to fix and harder to trackdown. You could have a bad connection in the control board or possibly a problem with the draft inducer fan (also a bad connection possible). If the inducer doesn't run then nothing else will happen.
Cooling is an entirely different circuit and runs off different leads in the thermostat cable. The fan behavior you descibe doesn't eliminate any of the possibilities.
Do you have any more questions?