I have attached a technical service bulletin to this post which is bang on point. I would change your evaporator temperature sensor before buying a $500 part on the basis of what the mechanic "thinks".
I'm pretty certain about my evap temp sensor diagnosis from your description of the symptoms. I took the car to the dealer several times and told them I thought it was the evap temp sensor and they just ignored me. Finally I went to a different dealer and told them that--among other things including the harmonic balancer--they were to change the evap temp sensor. They did it, problem cured.
My symptoms were as follows. I first noticed the problem on holiday in France where the ambient temperature was higher than at home. Running on the autoroute with the canvas hood up, the car was getting very hot. With the air con controls set to "AUTO" and 18 deg C, no air at all was coming out of the vents, although I could hear the fan running and the air con display suggested the fan was running at full speed. Switching the air con to "EC" mode would cure the problem, temporarily. About ten minutes after switching out of "EC" mode, the problem would recur.
These are the posts I made on this forum about my problem:
A/C diagnostics help
R129 AC problem
A good way to check for sure is this. Leave the car parked over night. Go to the car and turn the ignition on, without firing the engine. Run the self diagnostic routine on the air con pushbutton control panel and check the evaporator temperature reading. If the sensor is faulty, you will have a logically impossible value:
1. Turn ignition on (without firing engine)
2. Set the air con temp to 72 deg F or 22 deg C
3. Press "REST" for more than 6 seconds
4. The left side of the display will alternately display "01" (which is the test number) and the cabin interior temperature (eg 72 deg F or whatever the temp in the car happens to be).
5. Press the fan speed "+" button to advance to the next test (eg "02").
6. The evaporator temperature is test number "05". The working temperature of the evaporator is between 2 deg C and 12 deg C. Obviously, if your car has been left overnight and has not been started, the value returned should be consistent with the ambient exterior temperature. If the evaporature temperature sensor is broken it will return a logically inconsistent value, eg 40 deg C.
The part is a very quick and easy change. I would try it before spending $500 on something the mechanic "thinks" might fix it.
The other tests on the push button control unit are great:
01: in car temp sensor
02: outside temp sensor
03: heater core temp sensor
05: evap temp sensor
06: ECT temp sensor
07: refrigerant pressure meassured in BAR
08: refrigerant temp sensor
21: engine speed
22: vehicle speed
There are loads more.