I'm in a business where we work on electronic systems that are installed in big buildings. We sell and service card access systems, fire alarm systems, security systems, CCTV systems, etc. I used to be a field tech, then I was a service manager, but now I'm in the business operations side of the business. I know that a lot of troubleshooting is really just trial and error. Sometimes you replace a part and it fixes the problem, other times it doesn't. In my business if a part didn't fix the problem, we either put the old part back in, or we just left the new one in but we didn't charge the customer. Of course, the part we just pulled out could then be re-used because it was still a good, functioning part. I know that isn't feasible with something like a wheel bearing, or a shock absorber, but it seems to me that the customer shouldn't have to pay retail markup for a part that didn't need to be replaced. Perhaps in a situation like that the shop should just ask the customer to cover the cost of the new part, and even give the customer the old part to keep on hand just in case the new one breaks 5 years from now.