The check engine light comes on most commonly because something is abnormal with the way the engine is performing.

 The purpose of the check engine light is to protect the CAT converter in the exhaust system in order keep the air clean.

  Common causes of check engine lights are ignition misfires and oxygen sensors. Oxygen sensors become sluggish and worn out, requiring replacement every 100K miles. Tune-ups and fuel injection service prevent misfires.