A defective oxygen sensor may or may not trigger the check engine light to illuminate. A condition known as a "Lazy Oxygen Sensor" may at times, specially on older model vehicles, not cause the check engine light to turn on. Fuel delivery may be disrupted because the oxygen sensor can not relay oxygen content data in the exhaust stream to the engine control computer fast enough for correct fuel delivery. Exhaust emissions will increase, yet no trouble code be set.
