I've ran the diagnostics and it says that the furnace is in full working operation. But when I set it to on it will not start up.

Try jumping the red and white wires (if wired according to rule of thumb) the red being the hot wire sending low voltage power from furnace to tstat and white being the wire to take power back to furnace telling it to heat. If all works OK then you probably have a bad tstat. You can also do the same thing at the furnace where tstat wires connect to furnace, there is possibility of a broken tstat wire between the furnace and the tstat,, all this done after you are sure that there is power to the furnace,, you didn`t say if you had gas/elect/or oil heat. Would be good to know this when trying to help you

If the red and white wires are wired together will it work

If the red and white are run according to rule of thumb,, YES,the red should be hot coming from furnace and the white sends signal back to furnace to run,(I think I said this before),, wired straight if furnace is OK it will run ,but it won`t cut itself off till you disconnect the wires

If red and white wires are tied together (at the thermostat) and the unit works, then you need a new thermostat. If the red and white wires are tied together at the furnace, then it may be a bad thermostat or thermostat wires between therm and furnace.


If you jump the wires by the therm, does the unit work? If it does, then you need a thermostat.