Have a 2000 Honda Accord 4 cylinder. Problem started last week. Turned a/c to cold, cold air not as cold as before, plus a loud hissing sound that seems to be coming from the passenger side, under the glove compartment. So, turned a/c off, and just use air vents (turned off the recycle air button). Hissing sound continues, and cold air comes out of whenever car is in motion, colder than having a/c on. Is the car in big trouble? I have smog check due soon too...Anyone knows what is happening or has experienced this situation?
It sounds to me like you probably have a so-called "vacuum" leak. The "hiss" you mentioned is a tell-tale sign, and your air conditioning mode controls (both hot/cold, duct selection, etc.) are controlled by vacuum lines under the hood. At least, everything I just said it true of my Chevy Blazer, so I assume your Honda could have similar engineering. Sometimes these are easy fixes---especially if you look around where you hear the hissing and see a little hose the width of a pencil disconnected, or perhaps really deteriorated.
These aren't exactly your car---but they're diagnosing similar symptoms.
(This one isn't as similar as you might think, as it amounted to a bad compressor, I don't think your problem is as dire.)
It sounds like you have a sticking A/C compressor clutch, with the exception of hissing from passenger side, I would think from driver side. But this is an easy check and worth doing. Your compressor might be turning with the engine and the hissing noise is the scraping of metal on metal down in the lower left(when sitting in the car) of your engine bay. Honda uses cables rather than vacuum for the vent and temp selectors. Some parts are fully electronic with little motors and stuff. So no hissing from vacuum is going to happen.
With your A/C off and engine on, look down at the compressor and see if both the clutch plate and pulley are spinning. Use a flashlight and keep your fingers out of the belts. The plate is on the end of the pulley (coming away from the engine) and is about 1 cm thick with 3 bolts sticking out of it an additional .5 cm. The clutch plate normally will be motionless when the A/C is off. If motionless is not the case, fix it before you start melting things, the induction coil (magnet) will go first, compressor second. There are tiny shims that can be added between the clutch plate and pulley to increase the clearance and fix this problem, should it be the problem. Good luck, and if you don't have time, you can take the A/C belt off. I am pretty sure that the A/C has a dedicated belt in your car, but check before doing it of course.
All information from personal experiences with my Civic and Sister's Accord.