why does my gas furnace exhaust constantly
There might be an error code on your electronic control board. It's located in the lower half of the furnace. Sometimes there's a little plastic window that you can look through or you might have to take the cover off.

There'll be an LED on the board that will blink in a special sequence and give you a code number. There should be a label inside the cover or inside the wall of the furnace that tells you how to decipher the code.