09 Honda Civic AC

My ac has been switching from cold to hot air for over a year.

Initially, it would only switch to hot air while sitting idle at a red light and would straighten out if I turned the car off for an hour or so. The "specialized" ac mechanic I took it to told me they couldn't look at it until it stopped blowing cold air completely. Since it wasn't happening every day, I drove it around for about a year without too much inconvenience.

Over the past few months, it's been switching every day. In the morning, night, rainy or cool weather, the cold air will hold up pretty well and will usually last for maybe a half hour car ride. During sunny, hot weather, cold air will only blow for a minute or two at most. Turning the car off for an hour or two doesn't straighten it out anymore but sometimes if the car is run long enough with the hot air blowing, a sweet smell will come out of the vents and cold air will come back for a few minutes.

The Freon level has been checked numerous times. There is no leak. One mechanic said the compressor needs to be replaced. A second said the compressor along with all other parts are fine and suggested I replace the entire unit since they couldn't find a specific issue. A third mechanic also said the compressor seems fine and has enough pressure to work. That mechanic said I was just half a pound low on compressor gas and filling it up would fix it. I had to call back less than 15 minutes after leaving the garage because the ac switched back to hot air.

I have no idea what I should be asking mechanics or what to watch out for to make sure I'm not taken advantage of and can finally get the ac working.

Suggestions would be greatly appreciated!