I have a Goodman Furnace 075-3. It recently stopped blowing heat and would just blow cool air. I took the top cover off and confirmed that there was no flame. The gas valve to the furnace is turned on and the switch inside the top cover is also turned to the on position. There is a single light in the lower section that is on and solid which indicates normal operation. I turned all power off and then back on. There is silence for about 4-5 seconds. Then as long as I have the lower cover on, after 4-5 seconds I hear one click and then the main fan starts up and cool air is blowing throughout the house. No flame. The small exhaust fan never runs. I got a new exhaust fan and squirrel cage, but that did not make any difference. I unplugged the small rubber hose that connects to the squirrel cage and sucked on that to see if that would fix it. I could hear the switch turn on when I sucked on the small hose, but still no flame. The instructions say that the furnace doesn't have a pilot light. I couldn't find a reset button anywhere. I turned the thermostat to cool and the AC turns on and everything works fine. So it appears like the heat just won't turn on. I'm told that the exhaust fan should turn on first, but it never runs. I'm also told that the main fan shouldn't run if the exhaust fan doesn't turn on. But it does. I don't smell any gas. Any suggestions?

The fact that the main fan is running means that a limit switch is open. The Goodman furnace has at least three small manual reset switches in the upper compartment that could be tripped. They are black and the size of a very thick dime with a tiny red button in the middle. There could also be one or more in the lower compartment. Also there is a auto reset high limit in the upper compartment just above the burners that could be stuck open(a dirty filter can cause that switch to open repeatedly and fail).Hope this helps