First is to test the pressure in the AC system. That's simple enough. What's more likely is there's malfunction that didn't fully close the "vents" so the AC air can blow through the system, *IMHO*. My '96 LX has that problem, but quite minor. When I first start up the car, the vents blow warm air. Though a quick switch to AC for 5 seconds seems to work around that for now.
