Furnace fires up just fine, and fan spins but there is absolutely no airflow in duct work. Any ideas?

Last spring thermostat went out and I replaced it, worked great. A month later circuit board burned out, tech found that problem and replaced it (major pocketbook ouch) and everything worked fine. Month or so later burner fan quit working no issue it was now summer. Unplugged fan gave it a separate power source and it worked. Plugged it back in and it now works.

Have since checked all power connections for main fan and it's there, believe relay in board is not sending message to fan. Luckily board has 1 year warranty so tech is coming out tomorrow. Sure hope my diagnosis is right. Will know more then.

If I'm reading this correctly or should I say understanding you correctly... your thermostat calls for heat --> inducer motor (exhaust motor) comes on --> furnace ignites --> and then your problem comes in with no blower fan to circulate air through the home... Yes... if so either the motor windings are bad or the control board is bad. My question is why is the control board keep going bad... you need a good troubleshooter, not a glorified pars replacer, to fix your problem permanently. Although the board is under warranty you are still going to pay a service call and labor charge for replacing it. Is the Company your using very reputable?

Im a bit confused. You say fan spins. If the blower is spinning at a good rate, you should have airflow. Or are you talking about the draft inducer "fan". If the main blower is running, and no air flow, you may have a plugged secondary heat exchanger (if a 90+ unit) or a plugged evap coil if you have central a/c. This would also explain the board burning up, as if the coils are plugged, the evap coil will ice over and drip water down the unit and possibly onto the control board. Like jones said, you need a good tech in there to figure out what is happening. Sounds like it getting a bit spendy at this point. Certainly don't want to be replacing boards every couple months. Please let us know what the service guy finds.