Carrier model 58mta 9300TS: The furnace will run sometimes it seems like forever without shutting down.. Furnace was installed 03/2004 it just started acting this way lately I have installed a new digital 2 stage thermostat just last week it still acting up... does not want come out of 1st stage of heat anywhere from 45mins. To 4 hours... Any thoughts anybody . Thanks N-BY-NW

Check to see if there are any codes being posted by the circuit board. Usually if there is a problem codes will be shown in the number of blinks of the lite on the board. Also after you get the number of blinks there is a legend on the back of the blower door that will tell you what they mean. Post the number of blinks and the legend statement.

Your owners manual is here

http://67.52.177.135/Images/upload/products/performance%2093%20gas%20furnace/om1.pdf

The furnace should be equipped with a diagnostic LED that will blink/flash if a malfunction occurs. If it is a variable-speed unit, the circuit board will store faults in memory for later retrieval. The service label on the unit (usually found inside the front panel) should give a description of various faults and there respective remedies, as well as a description of the procedure for retrieving stored faults if you have a variable-speed unit. What you describe sound like the higher stage isn't working or isn't being activated.