1999 Buick Century Engine Light, Low Coolant, Overheating

First, has your mechanic even checked anything or assuming the problems? If the check engine light is on, then there is a code stored in the computer that will identify where the problem is coming from. If it is a misfire, it needs to be fixed before it causes other problems.

As far as the overheating, I would start with the thermostat to make sure it isn't sticking shut. Also, let me know if the car overheats at highway speeds or only intown driving.

Joe