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Service engine light back on

Tiny
NISSANWOMAN
  • MEMBER
  • 2001 NISSAN ALTIMA
  • 173,500 MILES
The P0420 code started coming on in my car listed above (GXE model) last September, after I filled up with gas at the grocery store. Nissan adjusted the gas cap and erased the code. The car passed emissions on the second try, but the service engine soon light returned within a couple of days. The other day (June 26), Nissan installed the new after market O2 sensor I purchased at O'Reilly's, because Nissan no longer supplies the part. The code was cleared but the light returned again the following day. Should I drive it a bit longer before trying to fix something else? My car doesn't have any of the classic symptoms of a bad catalytic converter. Could it be that more than one sensor needs replacing? And if so, would all of those codes be indicated at the same time?
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Friday, June 28th, 2019 AT 2:17 PM

1 Reply

Tiny
STEVE W.
  • MECHANIC
  • 13,011 POSTS
Welcome to 2CarPros. P0420 means the catalytic converter is operating below the proper thresholds. The catch is that those thresholds are so that the code will set long before the converter actually shows any signs of failure.
The only real way to test it would be to use a scan tool and watch the rear O2 sensor signal to see what it is doing. Being it required a second test to pass I would bet the converter needs replacement.
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Friday, June 28th, 2019 AT 2:45 PM

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