Most workshops make their money on parts, and that shapes what they do with a fault they cannot see: they fit the most likely part and see whether it helps. Sometimes it helps. The customer who has already bought three parts for one fault has paid for that method.
Here the thing being sold is a block of diagnostic time and a written answer. We measure, we eliminate, and at the end of it you have a named fault. Then you get one price for putting it right and nothing is ordered until you say yes.
It means the first invoice sometimes has no part on it at all. It also means we have no reason to want your fault to be an expensive component, and about one time in six the answer is an earth strap, a connector cleaned or a relay, which costs almost nothing and is the answer three garages before us could not afford to find.
If the fault turns out to belong to somebody else, mechanical rather than electrical, you pay for the diagnosis and we write down who to see.