SES light on, 24 hours in shop and no fix, HELP

geddy lee

My SES light came on. I figured it would probably be the gas cap or the evap control valve. I took it to my regular place ( not a Nissan dealer) who did alot of work on my 350Z and some work on my 2009 370Z. I called them after they had it for 6 hours. They said they would need more time to diagnose the problem. They kept it overnight. The next day, after 4 hours, thay finally call me to say they " THINK " it is a air /fuel mixture sensor located just befoe the cat. They replaced it after stripping the threads and then re-threading and installing the sensor. So I go and pick it up after paying $ 700.00 dollars for everything. The next morning on the way to work the f--king SES light comes on again. I go back to the place and they plug it in again and then tell me they will need another day with the car to go further into the diagnostics to determine if something else is wrong. After another 8 hours, they tell me they have recalibrated the whole sensor system and that I would have to drive it awhile so that everything re-boots. The SES light was now off when I picked it up. After about 30 miles of driving it has now come back on. I originally dropped the car off on 5/12. It was due for Delaware inspection on the 15th. Now it is the 17th and I am driving an unregistered car because they told me if the SES light was on, it would fail inspection. I don't know what the f--k to do, try to take it through inspection on Monday with the light on , or say f--k it and take it to a Nissan dealer.
I really need your guys help with this. The car is running great but I just don't understand why they could not IDENTIFY and fix the problem. I love the car and have never encountered a problem like this. I mean isn't it supposed to be that you simply pull the code and fix the f--king problem???
Thanks in advance