I am assuming that you have a V6 being there are two O2 sensors on your vehicle. It only takes one O2 sensor to trip the code. There is a way to test the 02 to get a reading on see if it is good or not. The mechanic is most likely replacing them both for PM ( Preventive Maintenence). Why replace one and maybe have you come back in a week or a month with the same problem when the other one goes bad next ?Kinda like if you have a bad spark plug . Wouldnt it make more sense to replace all of the plugs ? Or just the bad one? Hope this helps