The fact is that O2 sensors do lose efficiency as they age with high mileage. They may noy be throwing a CEL but they will negatively affect the fuel mileage and overall efficiency of the engine. I typically replace mine with factory original sensors (Motorcraft in my Fords) when the vehicle reaches 100,000 miles.

The following article gives some good insight and information:

http://www.obdii.com/articles/Unders...n_Sensors.html

 

They do age, but unless they're throwing a code, or fuel mileage drops off, or the car fails an emissions inspection for that reason there isn't any hard reason to replace them.

O2 sensor response usually gets a bit "lazy" before they give up entirely, and this is something that even most older scantools (like mine) can pick up straight out of the OBDII port.


Norm