Yeah that's most shops man, unfortunately. If they don't know, they just guess and charge you.
It's sad to hear about dealers working this way, though. All of those special tools and procedures to diagnose problems properly and they throw parts at it like redneck joe down the street... and charge twice as much.
Makes me glad I work on my own car.
2003 Grand Prix GTP.... the journey begins again....
E85 Self Tune with HPT, 80# Injectors, Gen V 3.4 pulley, SD Headers, AL605 Plugs