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Q: Why is my oxygen sensor failing every 30K? on 1998 Nissan 240SX

I have an 1998 240SX SE Auto. Ever since I put 60K miles on the odometer, I've been getting frequent downstream Oxygen Sensor error codes via MIL. The service life from each replacement unit averages 30K miles. Presently, I have 128K on the car.

About two years ago, I had error code for a bad catalyic converter (I believe it was code 420). I had a new cat put in and a vaccum leak addressed. I noticed something strange after the repair, a decreased of 20%-25% fuel economy. I swapped out the air filter, no increase. 2 months later, I get another two error codes for a bad downstream O2 sensor. Again, I replaced it and no increase in MPG.

Present day, I barely passed a section of my smog (ASM Emission Test). HC readings were 57 ppm @15MPH + 30 ppm @25MPH. CA law allows a maximum of 58ppm/42PPM. Technican said it may be a bad cat or o2 sensor.

What is causing this routine failure in my O2 sensors?
Do I need to replace other routine emissions equipment (i.e., upstream oxygen sensor or pcv valve)?
What is causing my degradating MPG?
What can I do to save my replacement cat?

*note: pcv and upstream oxygen sensor have never been replaced.
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I'd want to see data wile the car is running. When were the upstream sensors changed? What brand rear sensors did you buy? What does the computer think the engine temp is? Verify it as well.
Upstream sensor was never changed. I can confirm downstream sensor error code p0139 and possibly p0141. Brand of sensors were ac delco, bosche, and denso. Will need to verify engine temp with a tech.
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The ASM test isn't as accurate as some would like to believe. Lots of things can impact engine and catalyst efficiency and the loaded mode that the state chose can sometimes fall into a area where the system isn't quite as clean as it is during other times.

Are you installing O.E. sensors? (Nissan) When the sensors are failing do you recall specifically what code was setting?

As far as the upstream sensor goes, Mode 5 data would show the maximum and minimum voltages the sensor produces, and how fast the switch occurs from lean to rich and rich to lean. If those tests were just passing, then the sensor is degrading and getting sluggish and that could make a difference. Otherwise, a tech will need to test and evaluate your car which would include monitoring fuel trim, ignition timing, knock retard etc.
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