I agree with your premise. The customer shouldn't pay for guesswork. My problem was intermittent and did not generate an error code. Each "fix" solved the problem for a while before it returned. The car gad 150,000 miles on it, which is the time that many of the replaced parts reach the end of the dependability cycle. I know my mechanics well because they work on my other cars. My flakey set of problems are not unique to the Volvo 740 with the Regina engine management system. Every part they replaced was a logical act considering the symptoms that presented.
Using Gunk Fuel System cleaner abated the symptoms for a while, which led us closer to a solution that involved the fuel system and the engine management system.
Had an error code indicated the source of the trouble in the first instance, the fix would have ben simple. The mechanics did not charge me for unsuccessful efforts. I am just glad we all persisted because my Volvo runs flawlessly and I intent to keep it- and my independent mechanics-for another 100,000 miles or so.