When I turn the fan on either auto (temp controled) or on so it just circulates air it just buzzs but does not blow. How can I troubleshoot this?

Blower motor have what amounts to 2 sets of windings in the motor. It takes more juice to get the fan started rolling from a dead stop so it sounds like your starter winding is bad. This is not the safest thing but you can chcek this by shuttting off the fan, open the compartment. Have a second person trip.by-pass the compart door sfaety switch. You reach inside with a stick or wooden rod and spin the fan, just as you spin it and get your hand clear have the other person flip the switch on. Again, this is not a sdafe approach for a DIYer so be very careful if you try it. If the motor runs the fan shut it off and try to restart with the normal process. No spin after thuis and it's the windings. Get a new motor or price to have the old rewond, usually not cost effective