I had that same issue a few years ago, furnace would run all day and blow cold air, wouldnt shut off unless i turned it off, called repair man and took two weeks to diagnose and fix, first time he said it was thermostat, replaced that, nope! didnt fix it, said it was the board and ordered that, nope didnt fix it!, came back over and took the cover off, used his screwdriver to tap on this and that, then took some rubber hose off and sucked on it like he was ciphoning gas from a car then blew in to and PRESTO! out comes some water,,furnace worked fine after that.

BUT I WAS SURE PI$$ED OFF!! took em two weeks to figure that out!!