Hello and thank you for choosing Just Answer.
The fact that the light comes on indicates that the computer has stored a faulty code. When the mechanic fixed the problem and turned off the light, the light may have come on for a different reason.
I would suggest that you have the code scanned and fix to problem to permanently turn the light off. If you take the car to Autozone they can scan the code for you at no charge. They will give you the code along with a definition of the code.
I do not get paid until you press the ACCEPT button, please press ACCEPT. Feedback and Bonus is highly appreciated!