I have a Goodman GMT045-3 unit and I am having trouble with the blower kicking on. When the heat kicks on, the exhaust fan starts, the ignitor starts, gas flows and lights, but the blower never turns on. The blower will also not turn on if the thermostat fan switch is set to ON. BUT, if I switch it to the air conditioner the blower will turn on automatically.



What part(s) need to be replaced? Is this the board?

Sounds like a relay on the circuit board. I am not sure about Goodman but some of the relays are plug in type and you do not have to replace the complete board

How long does the gas stay on if it blower doesn't come on? If it stays on a minute or 2, likely it is the relay or other control problem. If it goes off in a few seconds, it could be the flame detector. Since trouble shooting the flame detector is covered in the sticky at the top of the heating forum, I won't try to cover it.