I once had a car that had lost almost all power; it was scary attempting to pull out into any kind of traffic. Took it to my local shop for a $75 diagnostic, and when I came back they told me to try "....(several things that I already had tried or replaced)". But, they were the mechanics, so I did them again. Wasted another $60 in parts, no change.
I was so pissed off at wasting $75 for the diagnostic, that I took the car back a few days later, with a letter I handed to the manager, that said something like "...Please don't give me your opinions about what is wrong with my car; I can get opinions for free, lots of them. I even have my own already. I'm not paying for opinions! You're the professionals with the professional tools to properly diagnose my car's problems, yes? If you can't professionally complete this task, keep your opinions to yourself, tell me you don't know, and I'll take my car and my business elsewhere. Please call me with any questions..."
They called me later that afternoon and said the car was fixed, and had lots of power, like new. They were right! It had never run better, and I know that my letter made the difference. And there was no additional charge! Moral Of This Story: Don't tolerate second-rate work that you paid for!
BTW, I once worked at a Precision Tune many years ago, and they wanted to get me, with no previous auto repair experience, ASE certified, which required (I think) a few years of previous auto repair experience. When I asked the owner about that requirement, which I failed, he told me, "There are ways around that." Shortly afterward, I too had an ASE certificate hanging on the wall in the customer waiting room, with my name on it. So understand; those ASE certs hanging on the walls in the shop may not mean anything; the one with my name sure didn't!