There are nigh on 1000 error codes that can potentially cause the engine management light (EML) to come on.
The technician will need to connect to the vehicles ECU’s in order to run full diagnostics to initially bring up the error codes and then to investigate deeper as to their root cause, monitoring areas for live data, history and other associated data.
Please note that diagnostics are very different from an error code read that has been done from a small simple plug in reader that just brings up codes.
These units have their place, but cannot be solely relied upon to give the reason for the root cause of problems. Diagnostic equipment costs thousands of pounds and that is why there is a charge for using them and the time required to do so.