Need to be more specific. Whats the model, year, mileage, engine?

Is there any loss of power when the CEL comes on? Studdering or anything? What are you doing when it comes on(taking off, breaking, cruising)?

98 Contour SVT (soon to be reborn) 2000 GMC Envoy... Screw you guys, I like it. 89' Mustang 5.0 (Just bolt on's... for now)