Thanks....However now I am getting a code 1 blink, for limit switch is open, how do I correct this????

I have a Nordyne M1Mb 056A BW and just recently replaced the thermostat, but when I did the furnace stays on after the click when it reaches temp. I reinstalled the old one, and it does the same thing. It comes on fine, then lights, then blows, when temp. is reached, it clicks at thermostat, then bl...