Contiminants from normal conbustion and oil ash accumulate on the sensing element. This reduces This reduces the sensor ’s ability to respond quickly to changes in the air/fuel mixture. The sensor slows down and becomes "sluggish." At the same time, the sensor’s output voltage may not be as high as it once was, giving the false impression that the air/fuel mixture is leaner than it actually is. The result can be a richer-than-normal air/fuel mixture under various operating conditions that causes fuel consumption and emissions to rise.