The Check Engine Light may illuminate because a component of the oxygen sensor stops working. As a result the engine computer is unable to determine the proper ratio of air to fuel for the engine. Replacing the failed oxygen sensor should correct this concern.
On higher mileage vehicles, an engine misfire may develop and/or the Check Engine Light may illuminate due to a failed ignition coil. It is not uncommon to replace all the coils when the first one fails in order to prevent return trips to the repair shop.