o2 sensor questions

I have read a lot of posts here referring to the o2 sensor. I understand what these devices measure when working properly, but had some general questions.
Do these devices have a finite life? If so, is it a number of years installed in the car or does it more depend on the number of miles on the engine? Or both. How long can they last? Should it be periodically replaced to keep the engine running optimally? When these devices fail, do they tend to tell the engine management to lean out the mixture or enrich it? Thanks for your help JB 88 300E 99 E55
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Average life span was 60K miles.

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The sensor does wear out. In early 80's MB's, they built a switch into the odometer to activate the O2 sensor light at approx 60K miles and there was no reset, so servicing involved replacing the sensor and REMOVING the warning bulb from the instrument cluster. A failed sensor in a Bosch CIS system would result in "limp home" mode and the car would run very rich, not lean.
With upgrades in the monitoring systems, the O2 sensor light became resettable and part of the OBD codes.
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I recently changed the original sensor of my 230E 92, with 90k miles. Idle, acceleration and emissions improved. The most interesting effect however, was on fuel economy, it improved near 7%, in my case enough to amortize the sensor cost within a year. I did it myself with a special wrench.

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re replacements.... I tried to attribute engine performance woes to my oxygen sensor since it was 250k km old. wisdom is to replace around 100k km. I learned how to test and found my sensor to be just fine... Turned out my problems were caused by the electric fuel distribution valve (EHA??) as well as the oil seals. So, O2 sensors can last a long time
cheers, guenter

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