My two penneth and personal experience, all too often faults are missed diagnosed and new parts fitted, this goes on until the fault is fixed through spare part surgery or the technician calls us for assistance, often with out removing the new part that didn't fix it.
An example, vehicle breaks down, engine just stopped working, technican attends the vehicle, plugs in his diagnostic computer. This shows him fault codes that say that all six unit pumps have failed, arranges recovery to the workshop, orders six new pumps and fits them, but it still won't start. Many hours later after much head scratching 5 gallon of diesel was put in the tank, it started and run a treat. The reason the engine stopped was because it ran out of fuel, the tank sender was faulty. Dealer claimed for the unit pumps and sender plus labour, needless to say this didn't add up so the claim was rejected and investigated.
I could go on quoting issues for ever, but I won't. Oh, for the record, I work for a premium brand Car and Truck manufacturer, in the truck technical department where I advise/assist dealer technicans with technical issues and faults they are unable to solve for themselves, on occassion this requires on site visits.
Rant over
Regards
Stuart
Remember, keep the greasy side down!