My mileage has dropped from an average of 28mpg to 25 to 26 over the past year or so. I replaced my O2 sensor about 150K miles ago.

Thanks.
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'86 sedan with 443,000 miles on the original engine and automatic transmission

The Bentley manual describes how to test the O2 sensor to see if it needs to be replaced. If you don't have one, let us know and someone will post the instructions.
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Mike F - 1984 244 DL - 285,000 miles
Original engine, transmission, drive train, starter
Undergoing reconstructive surgery with POR-15

I do have a Bentley manual and will look at it today. Thanks for the direction.
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'86 sedan with 443,000 miles on the original engine and automatic transmission

Few days ago I happened to ask my mechanic same question-his answer was basically every brick is different. he has an older brick with a touch over a million miles (really) on it-orginal sensor still working fine, and then another might go out in 15K-50k--etc. If you think yours is acting up just have it checked.

Bosch had (or has) a web page with a graphic taking you through the evolution of oxygen sensors and the increase in lifespan. Check the first reference cited on this Wikipedia page.

It also showed a shift in their replacement recommendation interval. The first ones, unheated, were supposed to last 30K, and the later heated sensors indefinitely. Last I looked, they changed "indefinitely" to 100K, or something like that, for the heated sensors.

I interpreted the revision as a concession to allow for the mileage-based changes an engine undergoes which result in contamination of oxygen sensors, that otherwise would have lasted "forever."

Personally, I do nothing in the way of proactive oxygen sensor maintenance. It will take a reduction of fuel mileage, a noticeable performance problem, a flunked emission test, or an OBD code set, to bring my attention to the lambda sond.
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Art Benstein near Baltimore

A dog teaches a boy fidelity, perseverance, and to turn around three times before lying down. -Robert Benchley

there's a good chance you need a new 1. 150k is ALOT of miles. before that design with the heater, we replaced them every 30k. i think volvo says every 60k after 85, but they seemed to work for longer than that. good luck, chuck.