A/C system

I drive my car for long trips and then turn off the A/C and the car. I come back to the to make another trip thirty minutes later and the A/C fan works but the system only blows hot air now or room temperature. The air coming out smells a bit like soda pop bubbles and gives me a headache. Tested system with a mechanic and compressor is not come on for the second trip but does operate during the first trip. As long a the system starts working when it is cold first it works, but when the system is hot still after operation it will not come back on. Have tried turning off and on repeatedly and that does not work. The soda pop when hot air is come out not when cold. Is the transducer the problem or a senor for pressure?