When the “service engine soon” light on your Cadillac comes on it means that there is a problem with one of the sensors related to the emissions system–primarily the oxygen sensors. These sensors monitor contaminants in the exhaust coming out of your Cadillac. Because Cadillac uses high-displacement engines in their vehicles, they produce a lot of pollution. These sensors help to optimize fuel economy and thus reduce emissions. When you’ve fixed the underlying problem that caused the light to come on, you’ll need to reset the “service engine soon” light.
Open the hood and disconnect the negative battery cable clamp from the negative battery terminal using a socket wrench. Turn the nut on the cable clamp counterclockwise with the wrench to loosen it.