Hi I have a day & night furnace not heating, I took a class in 2002 and learn some but have not practice much and now my house is cold and I like to fix it myself.
The unit would not heat up so I opened the unit up and found a code for flame roll out sensor bad so I replaced it same rating 350 as the old one. Cleaned the flame sensor, and restarted the unit the unit came on but did not turn off so I unpledged and restarted to reset now the unit is displaying a 31 code does anybody know what could be the problem with this unit please help

Have you checked the inlet and exhaust for blockage? Does the blower fan come on?