The flame sensor is a safety mechanism, which determines if the furnace will come on or not. If your Goodman furnace operates sporadically or not at all, inspecting the flame sensor is a good place to start your troubleshooting efforts since an accumulation of dirt or dust on the sensor will prevent it from detecting the flame.
Turn off the furnace. On the thermostat, select the "Off" setting, and also turn off the heating unit at the circuit breaker. To eliminate the possibility of electrocution or fire, there should be no power going to the furnace while you're attempting troubleshooting efforts.
Find the flame sensor in the furnace. Open the furnace door to locate the sensor. The flame sensor is usually a gray or metal-coated bar with a white base. It is approximately 3 inches long, maybe an inch longer. It will be located in front of -- or in close proximity to -- the gas burners.
Remove the flame sensor screws. There will be one screw, possibly two, attaching the sensor to the furnace. You don't need to unplug the flame sensor, just take the screws out to remove it.
Inspect the sensor for dirt or corrosion. Use an object with a metal tip -- like a screwdriver -- to gently scrape away any buildup. You can also use steel wool, emery cloth or sandpaper to remove any debris.
Put the flame sensor back in place. Tighten the screws and close the furnace door. Turn the furnace back on at the breaker and at the thermostat. Allow a few minutes for the furnace to come on.