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
2 answers
Try filling out this diagnostic sheet next time you take the car in to the shop, it may help the mechanic find the problem.
http://repairpal.com/blog/Heating-And-Cooling-Issues
Open Hood and check for Vaccum leaks.