Sometimes, the hardest part of being a tech is dealing with the customer. Customers always have expectations, some reasonable and some not, and we have to manage these on top of performing our diagnostics and repairs.
A large part of being a real technician is knowing when to trust your own expertise over customer demands. This struggle generally manifests in two ways:
1. The customer has their own diagnosis that they're sure is correct. We've all encountered this before. Something along the lines of, "I know it's the belt -- I just need you to come out and change it." The moment you start to put any stock in this kind of "diagnosis", you're already on the wrong track.
You're the technician, you're the one who knows how to determine the failure. That's part of your job description, and if the customer isn't happy with that, then that's your cue to back out of the job. After all, if the customer knows exactly what the problem is, they're the one who should be doing the repair...
2. The customer thinks the repair should cost less. A similarly tricky situation. You've done your troubleshooting, found the problem, and given the quote for the job. But the customer just doesn't think the repair should cost that much. Maybe they even tell you to find a less expensive, generic part for the repair, just to cut the price down.
Again, as the technician, you know exactly how much time and labor goes into each repair. You know that OEM parts will almost always make a more reliable repair than generic ones, even if they do cost more. And of course, you know the cost of running your business and how much you need to charge to make it profitable. These are facts -- they aren't for the customer to debate.
The bottom line: it's your job to know what the right repair is and how much it costs. If the customer won't trust your judgement, then you either need to convince them of your credibility or collect your service call fee and leave them to find the trained monkey they're clearly looking for. Don't be cowed by the customer -- you're the tech. Own it.