An ODB reader can be a great tool. We keep an older one in our Ford Ranger TV and a more modern ODB II/ECR unit in my shop. They can help you pinpoint certain problems, but they're not a panacea. A "misfire on cylinder 1," could, for example, be caused by a loose spark plug wire, fouled plugs, or a bad capacitor in the ignition circuit, amongst a host of other causes, and knowing your transmission is overheated is one thing, knowing why is another. You still need a mechanic's eye, ear, and experience to find the problem. I've done a lot of mechanicing, but it's not my profession, so I still miss things. For me the ODB reader is a win some/loose some proposition.
This was highlighted a few months back. I forget the code, but our Santa Fe was acting up, accelerating and decelerating randomly without our changing the accelerator pedal position; the ODB reader code was for something like a "throttle position out of range" code. When I went online I found the most obvious reason for such a code was the throttle position sensor wearing out, so I replaced it for $50 or so,and the problem went away.
For a while. Then it came back. I took it to our mechanic, and he found a frayed cruise control throttle cable. The throttle position sensor had nothing to do with our problem at all.
The reason why the code went away? The car stores recent throttle position information in memory and uses it and other information to calculate how much fuel to send through the fuel injectors. When I disconnected the battery
, the historical information was lost, so the faulty readings had been erased and the anomalous data hadn't yet reappeared. Then the cruise control cable hung up again for a moment, the erroneous data re-appeared and the problem recurred.