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1998 Nissan 240SX Questions

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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.
My analog speedometer fails, it's jumping all the time, like if for a few millisecond there's no signal or something, but it only happens when it's a hot day, as the temperature starts to fall down then the speedo works just fine. The other instrument gauges in the dashboard are working just fine.
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