Me, I'd pull a spark plug or two to see if its running rich -- maybe the onboard chip has got the air/fuel mix wrong, or a fuel injector is bad, or ... you get the idea.
For the tech to read the data and shrug his or her shoulders and say nothing is wrong is being a little lazy, IMHO.
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I think you fail to understand what the technician can see and interpret.
A good technician can drive the vehicle and tell whether it has the proper amount of power, whether the transmission or clutch is slipping, and whether the transmission shifts through all the gears properly.
A good technician can look at the data stream and see whether the vehicle is running rich; he doesn't need to pull a spark plug to do that. In the data stream, the technician will see values for the oxygen sensor readings and the fuel trims. The oxygen sensors, combined with the programming of the engine control module, control the fuel mixture. So, to determine if the vehicle is controlling the mixture properly, the technician looks to see if the front oxygen sensors cycle properly. The technician looks at the fuel trims to see that the computer isn't unexpectedly providing significantly more or less fuel than expected for the given operating conditions.
Excessively large fuel trims (in either direction) indicate problems with oxygen sensors, unexpected air or fuel leakage into the intake/combustion chamber, or possibly other problems.
And this isn't the limit of the data the technician can see. A good technician doesn't just read the data; he interprets the data to see whether it appears correct. So, no it's not lazy. It's the technician performing a task to approach a problem in a logical and efficient manner. Suppose you were footing the bill? Would you want to continue paying the technician to pull stuff apart until he found something? How much would you spend without the technician finding anything before you were willing to give up? Would your engine be in pieces on the floor? The reality is that checking the data stream, combined with a test drive, *is* an effective way to determine whether a vehicle is running properly.
I give some kudos to this customer. She actually knows her fuel economy. Most of the customers who complain about their fuel economy have no idea what it is and many are confused about how to compute it. It's sad, really. And I agree this poster's fuel economy is low enough to give some pause. But I know neither the customer's driving style nor driving conditions. It's enough that I'd look pretty hard at all the data. I'd also want some more information regarding what is involved in a typical daily trip for this customer-- especially if I found nothing wrong. Does she warm the vehicle prior to starting a trip? How short are the trips? In what sort of traffic are the trips taken?
Just like any other problem, this one needs to be aproached logically in order to find a resolution.