Carrier Weathermaker 8000 not kicking in as it should

For about a week now, our furnace hasn't been working as it should. We woke up and found the house cold. We normally have the T/S set at 68. The house was 62. We tried upping the temp on the T/S and it did not kick on the furnace. So we turned off the T/S and then back on. The furnace kicked in. Then a few hours later the house was at 66 and the furnace was not automatically kicking on. We have played with it, turning it on and off (if we put the fan to the ON position, it will continuously run). We thought it was the flame sensor so hubby took it out today and cleaned it with sand paper. Well, we thought we were doing fine but now the house is cool again......and the furnance will not turn on by itself. Any help? MOney it tight so we need to try and fix this and it is Michigan so.....

Thanks for any advice!

 

Did it stop, or you had the set point down lower?

The house then did come up to temp when you left the fan switch ON?

Plan on replacing that soon! Sandpaper should not been used. The green scotchbrite is what should of been used.

Was the fan switch turned back to AUTO or left it in ON?