Here's where a "mechanic" did the computer's bidding without knowing what he was doing:
I bought a used car from a local used car dealer that developed a bad catalytic converter. The OBDII codes said so. I took it to the Mazda dealer for warranty repair. The kid who checked it out (had more hardware on/in his head than in his toolbox) said that it needed new plugs, and valve cover gasket, because that's what his computer said to do. The used car dealer obliged, and of course that didn't repair the problem. The car wasn't misfiring, the cat was bad. Second trip to the dealer, the service manager came to me and asked me if I knew the cat was bad. I said yes, that's why I'm here. He said that it would cost me about $800 to replace the cat
I informed him that it was a warranty repair, as the car was well within the federal limits. They were going to charge me, then submit a claim to Mazda. Just another bad dealer experience for me. And on top of everything, they charged the used car dealer $80 to diagnose the problem.
Last time I ever went near that Mazda dealer. The car is great, dealer support, not so much.
Most dealers seem to have one mechanic that sort of knows his way around, and a bunch of monkeys.