Most of the in-depth codes are proprietary, but at the same time, things like mass airflow sensor info and throttle position sensors are under the standard specs for OBD-II info. So even without paying the many thousands of dollars to Ford, etc for the info, you can usually get a good idea of what's going on.

Also, the check engine light is something that can display things that people would only consider tangentially related to the car's emissions system. Things like misfires, or even occasionally transmission problems, can also trigger this light.