Have HVAC Questions? Ask an HVAC Expert for Answers
You may not actually need a new thermostat, in most cases it is the furnaces internal controls that are on the fritz, not the thermostat, tell me please why you suspect the thermostat?
Follow the instructions on page 3, tell me if that is what you have or not, if it is what you have
take a photo of the thermostat wiring that is exposed when the front removed.
attach it here using the paper clip icon at the right end of the tool bar above, or the 'attach files' link to the left of your send button.
There is about 50% chance you wire, and program a replacement thermostat yourself if it turns out that you actually need the thermostat.