My Lennox G20 gas furnace ignites the pilot light and starts the main burners. The burners burn normally for about 2 minutes and shut down without the blower fan starting. If I manually set the blower fan to run continuously, the heater cycles on heats the house to the temperature set on the thermostat and shuts down.

I'm guessing there is a sensor or controller problem, probably which ever costs the most.

Sounds like the motor blower relay or the fan control is defective. This makes the limit control shout off the flames because the blower is not working. When you turn the fan to manual the blower runs and all is well.You might look in the sticky notes in the beginning of this board to see what those parts might look like. Or you can use your local HVAC contractor that deals in Lennox to see if they have the part you need.

I have a Lennox G20Q2E-75-7 furnace. The blower fan won't automatically turn on when we turn on the AC or fan only. It does automatically turn on when the furnace is on heat. Help please.

Sounds like a problem with the blower motor relay which on most later units is located on the circuit board in the furnace.

Your service manual

http://0190acd.netsolhost.com/manuals/G20_Series.pdf

So if that is the problem, we need to replace the BCC board in the furnace?

So if that is the problem, we need to replace the BCC board in the furnace?

IF that is the problem then more than likely the control circuit board is defective. Naturally the board should be tested to be sure it is defective before replacing if you have the knowledge of how to test the relay.

Another item to check is to make sure the thermostat is sending the signal to the board to start the fan motor when in the A/C mode. I always like to try all other possibilitys before replacing an expensive circuit board.

Possible sensor problem or maybe your motor, it has a capacitor on the side that may
Fail first... easy fix...