The time in diagnosing the car is separate from the time used in fixing the car.
Spending an hour on the car is more than enough to diagnose the problem. If I need extra time to find a fault, I will need to explain the customer in detail about the additional costs. My technician is appointed to diagnose the car; we are spending business hours in finding the fault with his car. That is an opportunity cost for us because we could be spending those hours fixing someone else’s car and earning money.
If an arrangement is made with the customer where the technician drives the car to and from work, and driving during lunch breaks then this will be ethically more acceptable. The equipment, installed in the car, will record and diagnose the problem. The time my technician spends this way is not a burden on business hours. In this case, the total cost would come up for only two hours. The technician spent the first hour diagnosing the problem and did not find the fault. After that, the arrangement is made with the customer that my specialist will keep the car to find the problem. He will be driving the car to and from business and during lunch breaks. When he finds a fault, it will take probably less than an hour to fix it. So one hour for diagnosing and the other hour for fixing the problem would be charged from the customer. This method seems more professional than any of the other scenarios.
What if you change every conceivable part that could cause this; would that be professional and ethical? If you cannot fix it, would it be more ethical to charge a nominal fee rather than what is due to you for services rendered?
Changing every conceivable part is neither ethical nor professional. This way the customer will have to pay for the time consumed and the cost of new parts. Charging a nominal fee or for the services rendered will be an enormous burden on the client. The technician can use a 'trial and error' method, making educated guesses what the problem could be and then change that particular part.