Have a Carrier Furnace and the blower won't shut off. My furnace will not heat since this has happened. Put in clean filter. Red LED reads 3 short and 3 long lights

3 short and 3 long do you have a sheet glued to the inside of the door/cover, the codes should be on it,

Sounds like the thermostate is stuck, not connected broken or fan switch is on constantly

Code 33 is a limit switch fault. You replaced the air filter - was the old one clogged? If so, the main limit switch may have been cycling the unit for some time, and could now be stuck open. There could also be a tripped auxiliary limit switch. If so, will require manual reset.