At the very least, I'd file a report with the BBB. It's very obvious the sensor had to be plugged in for you to drive the car to the dealership and it was very obvious that it was purposely unplugged. That means that they must have unplugged it.
Whether they unplugged it to do something and simply forgot to plug it back, or if they purposely did it, you cannot prove. But it's still a case of gross incompetence.
Ask for your money back, but don't expect it. If they refuse, simply tell them (not with hostility) that you will write a letter to the BBB about it. One letter will mean generall mean nothing to the BBB, but for all you know this guy has gotten caught in the past, and all the BBB is doing is waiting for one more.
But don't let it slide without doing something. Whether or not they did it on purpose, or whether it was incompetence or fraud, they're likely to do it again. And if they had done it to someone like myself, with no means of checking how correct their diagnostic was, they'd have swindled me for $3,800.
If you complain to the right people, if it was incompetence, they'll be more careful next time. If it was fraud, they'll think twice next time.
