I have a 1998 Honda Civic that my wife uses for commuting. With the onset of cold weather recently I discovered that the heater does not work when switched to bring in outside air. In other words, when it is set to recirculate inside air, it heats just fine, but as soon as I switch over to fresh air it starts to blow cold. This is all happening with the AC off. My mechanic does not know off the top of his head what the problem might be. I would sure appreciate any ideas or suggestions. Thanks in advance.

Puzzled in Lawrence, KS