MIL Light on Code P0420 on 2001 Nissan Sentra

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So, I know what the code is, but in order to get it repaired, I still needed to smog my car (knowing it will fail) with hopes that the test will tell me what needs to be fixed. I am in CA. Well, low and behold, my car passed all emissions, the Cat passed, everything except for the fact that the MIL is on. It will clear with the code clearing when hooked up to the OBDII, but within a few days comes back on. It first came on after a coil and plugs were replaced. Then it went off for a few days on its own before coming back on. Any clues as to what is causing the issue when all other functions are passing? BTW, I passed emission with flying colors (low numbers based on max allowed)
(1) Answer
It's a catalyst efficiency code - and it's measured by the engine's processor. It's not seeing enough of a change from the pre-cat oxygen sensor's readings and the after-cat oxygen sensor's readings.

Some manufacturers have processor reprogramming that can aid this reading, but it sounds like you may have had a true deterioration of the cat if you had plugs and coils at issue in the past. Having passed the smog numbers, at least, you can certainly try and hope for another alternative. Good luck.