Taking the analogy t to health…… a thermometer is an excellent diagnostic tool I can use to decide whether to take other actions like fever reducer, staying home, deciding whether so seek medical attention. It’s useful to me. An MRI machine may be very useful to a doctor but if I could have one in my home it would not be. It may be a great tool but I can’t use it to decide on my best course of action nor save money.
I suppose it depends on how much work you do yourself (and admittedly I do less than I used to), but I’ve never had such a readout change what I do. Even in the above example, addressing the error code was something I was going to have to have someone do and I have a shop trustworthy enough to point out when there is a potentially cheaper option for me
To be clear, I agree it’s important for mechanics to be able to read the codes, but disagree about consumers. I might do it since that’s gotten cheap and I like to see data but don’t expect it to be useful.
Basically if it’s easy enough to do myself, reading the code doesn’t usually help diagnose. If reading the code helps diagnose, the chances are tiny that it’s something I can work on. Obviously there are people who do a lot more of their own car repairs