Reading trouble codes means simply plug in a code reader and read the code. i.e. PO304 would mean a misfire on cylinder #4. Diagnosing the problem requires reading the code and determining why the cylinder is misfiring. It can mean diagnosing low compression, a bad spark plug, a shorted spark plug wire, a bad ignition coil, a bad fuel injector or a vacuum leak requiring specialized equipment and our time to test properly.
This is an example of just one trouble code. Each code will have its own set of possible causes. The diagnostic fee for S.E.S light is to cover our time to accurately test the components of the system that is failing.