OK what kind of a test did they do??? Did they run the car on a DYNE?? If they did they lunched your motor for shure. But if they did an obd2 scan for monitors they did not. My gut feeling is they messed you car UP badly. Scan for codes and post back. Po306 is cyl 6 as you pointed out. I would check compression and my gut feeling is the cylinder is in distress. Do you have any recourse to have them fix the car. They wacked it out and handed you the keys. Man their would be KNUCKLES FLYING on that one. What did the supervisor of the place say. Or for that matter the owner of the place. I would confront him and start asking questions. Goos luck DUDE.