I have a AirEase, Ulta V, Tech 80 furnace. My thermostat says its 66 the house I sent it to 70 and for the most part it wont keep the temp., it wont turn on and when I checked the code its blinking 6 & 4 flashes. According to the instructions it indicates that it is "locked out due to open limit". What would you suggest that I do to rectify this problem? I understand it may be a filter issue, but that's been changed 3 weeks ago. Please help.
Along with what harley rider said, make sure all of your vents are open, and there is no furniture blocking any return or supply vents. Make sure all of the return air grills are clean. Make sure that the blower actually turns on when you restart the furnace.
If you use a 1" fiberglass filter and change it regularly, there will still be a lot of dust that gets through over years of use. You can CAREFULLY vacuum the blower motor/wheel if it has a lot of dust on it.