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Hey everyone, any help here is appreciated. CEL went on a few weeks back, hooked up the code reader and it was throwing the P0320 code only. I did a quick google search and read up a bit on the upstream 02 sensor, and that a faulty sensor was likely the problem. Since the car has over 100k miles, I figured I'd start there so I ordered a replacement about 3-4 weeks ago. I've been driving it daily since and the car was (and still is) running great, no stalling or idle issues, and no noticeable drop in fuel economy. In fact, average MPG has actually gone up a bit since the CEL first came on.

Finally got around to replacing the sensor this afternoon and reset the ECU. Drove the car through a couple drive cycles and the CEL stayed off. Drove it a third time and the CEL came back on, now with code P0131, P0320 and P0420 all concurrently.

I'm somewhat handy but not a mechanic, so these don't point out anything obvious to me. Especially considering there hasn't been any change to the driveability of the car or fuel economy.

Anyone have any experience with this issue and/or any knowledge as to something obvious these codes could be?

Thank you in advance.
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