Where I live, it is common for car mechanics to help their customers who are planning to purchase a new used car by performing an OBD2 diagnostic test on the car in question, even if there are no dashboard warning lights.
What is the point of this? According to my understanding, if the vehicle’s self-diagnostic and reporting capability has detected an issue, it will show a warning light on the dashboard. Then, the OBD scan would be used to get the specific diagnostic trouble code for the issue.
I know that it is possible to mess with a vehicle and disable the dashboard warning lights. However, in that case, the lights do not come on when turning on the ignition, so I assume that this is easy to tell.