When my customers drive my customers chevy cavalier 2.2l 4cyl for more than about 10 to 15 mins 40mph or above, the check engine light turns on. Checked ecm for codes found none. Replaced Oxygen Sensor because it was all rusty anyways, but still didn't fix problem Car Diagnostic Tools. When my customers turn the car off and on again the check engine light wont go on again untill my customers drive 10 to 15 mins above 40mph. No idea what is wrong any help is greatly apprieciated.