Check engine light 1998 Chevrolet Cavalier

I have a 1998 chevrolet cavalier, 4 door automatic, The check engine
light has been on since I acquired the car 2 weeks ago. The mechanic hooked it up to the computer and got no codes and cleared the computer out and the check engine light went out until I drove it and then it came back on. The mechanic hooked it up again to the computer and he said
he was getting no codes but was told by someone to replace
the coolant sensor. He did that and the check engine light went out.
When I got the car and drove it about 1/2 mile the check engine light
came back on. When the light was out the car ran really smooth,
when the check engine light came back on the car started idling a
bit rougher. Does anyone have any suggestions? Their was a recall for the wiring harness for the ignition that I have since gotten taken care of and the mechanic is going to check it again but he hasn't had time yet.