[QUOTE=manny]A good O2 sensor can be observed by watching the " cross-counts ", i.e. how many times it switches from rich to lean, in a given time.
Duke 2.6 correctly pointed this out earlier.
As an O2 sensor ages, it will get " lazy ", i.e. instead of, say 10 cross-counts/second, it slows donw to, let's say 5 crosscounts.
This may not result in the illumination of the Check engine light, but can have an affect on emissions & fuel economy.
Bottom line..........it's up to you to decide. [/QUOTE

I believe ten "cross counts" is too fast. The typical "period", which would be two cross counts should be about one second. If it's slower than two seconds the sensor may be getting lazy.

The actual response time of the sensor is about a tenth of a second, but the system richens very slowly so it doens't get ahead of sensor response time.

This is my understanding based on reading about O2 sensors, but I'd welcome anyone's comments who has good info as I have found it difficult to find information on O2 sensor failure modes.

Slow response time is one and I also suspect that they may drift off calibration, but have not been able to confirm.

Duke