heating woes

["Titus, Brian (NJAOST)" <BTitus@njaost.ml.com>]

This morning my 885ksq (with automatic climate control) would not deliver
heat to the vents. Fan is blowing like crazy trying to make the requested
temperature, but only cold air is coming out of the vents. I believe the
system thinks it's calling for heat, because the air is not blowing out of
the dash (A/C) vents. Anyway, I checked the archive, and found many
references to failed heater control valves. I took a quick look at mine and
it seems intact; also the coolant in the expansion tank is at the right
level, so it doesn't seem as though there has been a major leak; engine temp
guage reads normal. Do the control valves tend to only fail
catastrophically, or do they simply go bad (get stuck), or does this sound
like something else (vacuum leak, bad heater core, etc)? Unfortunately, I'm
at work right now, so I can't really go out and test anything... :-(

Brian
885ksq, 106K miles