I'm am absolutely not an expert on the A/C on these cars, so I'm just guessing here. I know your model is equipped with one.
I have to go back and look at my manual to see where that sensor is located. I'll do some research on the A/C system as well and see what I can dig up for you.
My real concern is that I hope your mechanic is doing this work with the battery disconnected. The climate control unit (the one on the console with all of the switches) is expensive, and is prone to failure anyway as your car ages. We don't want to aggravate the situation by having him work on your car while the electrics are hot, as it won't take much on his part to fry the unit, or anything else on your dash.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car