Our furnace doesn't seem to be working properly. It won't come on when it should and it isn't blowing hot air when it should. The fan blows air. We are trying to change the thermostat to see if that is the problem and I am not sure how to choose the J2 jumper setting for a gas furnace. Do I use the black tab to cover the closet two prongs to the HG or do I cover the two prongs that are closer to the HE. The furnace seems to kick on for a minute then goes right back off.