I have a 2010 Holden Cruze. I recently had a problem with a drop in power when going uphill. I put the car in to be fixed explaining what the problem was. After some time, I was told that it was a diesel-particulate system problem. After some work and replacement sensors, I was then told I needed a new intercooler. I asked if this was definitely the problem to which I was told they were not certain but it was most likely and they would replace the intercooler and check after that. They did the work and told me all was good. I paid $2300 for that.
Five months later, and I had the same problem. At the next service, I was told I had a leaking boost-hose, oil around the injectors and 'O' rings. The repairer also recommended a MAF clean, injector clean and a timing belt kit. Am I stupid in thinking that they should have checked the boost hose, and injectors when they originally were looking for where the power failure originated?
It sounds to me, Michael, that your repairer is not being entirely scientific about the diagnosis procedure. Changing things because they might fix the problem is an absolute hiding to nowhere in modern cars, purely because there are so many sensors and systems that can cause all sorts of problems.
While your repairer is correct that a diesel-particulate filter problem can cause a loss of power, so can a faulty intercooler, a dirty MAF sensor, leaking injector O-rings and a leaking boost pipe. Okay, so those things have all been fixed, but which one was causing the problem?
The smart way to go with a vehicle like this is to plug it into a scanner and download all the fault information that has been logged by the on-board computer. Only once you know what components are dodgy can you make an informed decision about what bits and pieces to replace. Beyond that, you’re stabbing in the dark and forking out big dollars every time a mechanic says "let’s try this…".
So, no, you’re not stupid in thinking that all the possibilities should have been considered before work began, and I’d be having that very conversation with the workshop involved. It may be that all those components that have been replaced were, in fact, faulty, but replacing things until the problem goes away is often a very expensive way to tackle a problem.