Good Morning Gents,

I have been battling this issue for over a month now, I have replaced the MAF sensor and the Catalytic Converter/header this weekend to clear error codes to test for emissions this month. I have 05 tC with 140k miles. As i worked on replacing the Cat, I pulled the negative off the battery to clear the code. I took the car for a test run after install, and within 15 miles the CEL popped back on, and when driving around, at about 75 miles it turned off. I woke up this morning and went to work, and the CEL came back on again. Went to AutoZone and the P0420 is active. Looking through all the forums regarding this issue, it seems like an O2 sensor could be bad, which one should I replace first?

Thanks in advance

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Primary O2 sensor on header probably, $200+ from the dealership.
I have an extra one used, let me know if your interested.

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Like mention above. Primary O2 Sensor on the manifold.

Replacing MAF Sensor and Cat wouldn't solve that code. It's a sensor related issue.

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okay, so the secondary sensor would not have anything to do with this? I also noticed intermittent rough idle, the primary sensor screwed into header is the culprit. I will change it out!

 

did you install a new or used cat

 

Brand new cat, i have a stock car only modification was an injen CAI installed at 10k miles back in 2005

 

catalyst has a warranty of 8 years or 80k miles

 

1st- P0420 is for catalyst below threshold (meaning catalytic converter is not up to par). Primary sensor wont do jack, if anyrhing your secondary sensor could be bad not giving the computer correct readings.
2nd- Where did the replacement come from? If it says made in China it's junk, even a new catalytic converter will show as bad, there parts don't meet our requirements. Make sure it says OBDII compliant.

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Did you get it fixed what was the issue??

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Interested in this outcome as well. I have the same code and it's ____ing my off. My OEM cat only has 6k miles on it and there are no apparent signs of damages to the cat or entire manifold. Really don't want to replace the $400 manifold.

 

^^^ what he said

 

check this out.... if you still have the original cat , do this

 

I just had the same issue with mine and I had just put in Dezods Catted Header 3 month ago so I bought a new O2 and now 300 miles later light still off.
The P0420 code is for both sensors and the cat it's just finding which one is causing the CEL.

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The code is only for a: bad catalytic converter or b: bad secondary sensor (after catalytic converter, not before)

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Well I replaced the one be4 the cat and CEL light hasn't come back and I had the P0420 Code.

Code was coming on every 50 miles. I just kept resetting it thinking it would go away lol. So i gave up and I replaced the first sensor be4 the Cat and reset the Code. now I'm almost at 400 miles and no CEL (knock on wood lol)

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Which sensor, if I remember correctly our cars came with two catalytic converters.

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The one on the header

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Was your car running rich before changing the sensor?

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Not that I know of I haven't installed my wideband yet.

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