Since you already paid a mechanic and had it scanned did he at least tell you what he thinks is the problem?The check engine light is on because one of the sensors that communicates with the computer is not functioning. Most likely the air flow meter or air temperature sensor, needed for cold start ups so that the computer can adjust the air/fuel mixture to run smooth while cold. When car is warm all is fine, cold it needs a richer mixture. The mechanic should have at least given you an idea of what is required.