We have a 2005 Sentra. Once you put the air flow control in the defrost position, you cannot stop the compressor from cycling no matter what position you put the air flow control in. The only way to stop the compressor is to turn off the fan switch and leave it off or shut off the car and restart it. Even if you have the air flow control back in the vent position after the defrost setting with the fan switch off and turn the fan back on again, the compressor engages again. Unfortunately, the manual confirms this is the way the system works. That's ok for short trips, but if you were going a hundred miles or so and only wanted to use the defrost for a minute or two, you would have to pull over, shut off the engine, and restart the car with the air flow control in a non defrost position assuming you wanted to run the car with the fan switch on but no air conditioning once the defrost setting has been used.

Another member and I discussed a similar issue a few months ago. I'd put on the air conditioning (I have a 2005 Sentra too), and even if I turned off the AC button, it wouldn't always turn off. Turning the car off didn't always fully disengage the AC - when I'd start the car again, sometimes the cold air would still be on...I discussed it with my service dealer, and they seemed to think I was nuts!

If this is "normal", I would like to have an explanation from th engineers who designed this. IT JUST DON'T MAKE NO SENSE!!