“O2 sensors do wear out and eventually need to be replaced. The performance of the O2 sensor tends to diminish with age as contaminants accumulate on the sensor tip and gradually reduce it’s voltage. This is caused by substances such as lead, silicone, sulfur, oil ash and even some fuel additives. The sensor may also be damaged by water, road salt, oil and dirt. The best way to validate a O2 sensor is functioning normally is to check responsiveness to changes in the air/fuel mixture with an oscilloscope. if an oscilloscope is not available, the O2 sensor’s output voltage can be read with a scan tool or digital voltmeter, but the transitions are hard to see because the numbers fluctuate quite a bit. And it is another method using LM3914N dot/bar display driver.”