I could buy this statement if I hadn't seen examples where even with the "expensive computer equipment" the bloody garage (franchised main Volvo dealer) couldn't work out what was wrong - and kept believing the onboard computer/workshop computer without looking at the simple logic of it all. For the most part the expensive computers tell them nothing you couldn't work out with common sense and time (as in the case of the Volvo - a failled Lambda sensor - that the car went back no less then 8 times for the same fault - thankfully under warranty - and I suggested at the first visit it was the lambda sensor - but the computer said it was ok). An engine is a fairly simple beast at the best of times - just as time progresses - more and more of it has become a closed-loop control system rather than an open loop in days of old.
I struggle (tho someone will be along in a few ticks with a great example) how many faults you can't work out logically, or to think how many times the car would have been back to Volvo at 100+vat per hour for them to check the computer, have it say "emissions over limit" - check the lambda sensor by asking the computer if it was ok - to be met with the answer "yes" - and then moving on elsewhere.
They were Which Volvo specialist btw - not in east anglia is it?
Cheers Dan.