Resetting a check engine light requires completing a diagnostic test on car's computer, which varies up to $100, as of April 2015. Dealerships generally charge a higher cost to complete a diagnostic, while smaller shops may waive the fee if the driver has the required repairs done there.
A diagnostic requires plugging the cars computer into a special machine called a code reader. The machine checks any codes that cause the light to turn on in the dashboard. Once the code reader captures the codes, it sends the reading to the diagnostics machine.
The mechanic will check the diagnostics machine and tell the owner of the car what needs to be replaced in order to clear the code from the system. The mechanic can reset the engine light, but if the problem is not fixed, the computer will eventually turn light back on.