3 days ago I noticed cold are coming through the vents when the heater kicked on. I went down and Cleaned the inside and the filters. It worked for a little bit and then went back to blowing cold. The red light is blinking 4 times... open limit device. When I noticed it not working I have had it off It says open limit device. I have had it turned off for a night and day now. What do I need to replace and where are these reset buttons people are talking about?

I forgot to say it is a Trane XR80