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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.