Heater blows cold air on 2003 Ford Focus

My heater use to blow cold air just when I was stopped at stop lights, or just idling. Now, it blows cold air all the time. I took it to a mechanic, but they "could not duplicate problem." What could it be?

Asked by for the 2003 Ford Focus
Try filling out this diagnostic sheet next time you take the car in to the shop, it may help the mechanic find the problem.
1 more answer
Open Hood and check for Vaccum leaks.