sometimes even after a reset of the computer...it takes time for the computer to learn your upgrade and come up with a new fuel map...and in the meantime your car might run lean or misfire a cylinder leading to a check engine light, which won't go away until it clears itself after so many starts w/o problems or until you use a scanner to clear it. It's good to have an obd-ll scanner to be sure your car's ecu is taking the new upgrade ok.