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P0420 code

Tiny
MONDOFOX
  • MEMBER
  • 2007 PONTIAC G6
  • 2.4L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 147,000 MILES
Got the 0420 code, O2 sensors checked out okay, both on OBDII and with an ohm meter. Less than 50 degrees temperature difference between cat in and cat out. No lack of power, but fuel economy had slipped by about 10% over the past 50,000 miles and the exhaust manifold on the 2.4 had the universal crack between cylinder 2 and 3. Replaced the cat and manifold. Code didn't clear within 75 miles so I reset it. 200 miles later it again threw a P0420 code. Front part of cat housing temperature after warmup is 309F. Outlet end of the housing is 204F. No lack of power. No change in fuel economy. O2 sensors still OK on OBDII. Haven't resistance tested them again. Ideas?
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Sunday, October 11th, 2020 AT 9:55 PM

10 Replies

Tiny
MONDOFOX
  • MEMBER
  • 6 POSTS
Additional info: air cleaner element is fresh. Spark plugs were replaced 12,000 miles ago. No change in performance or economy from either. Cat temperature check was done 25 minutes after shutting down from a 60 mile drive. Warmed it up another 4 minutes before taking infrared temperature readings.
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Sunday, October 11th, 2020 AT 10:11 PM
Tiny
DANNY L
  • MECHANIC
  • 5,648 POSTS
Hello, I'm Danny.

That code is for the catalytic converter and not the oxygen sensor. Here are a few tutorials for you to view:

https://www.2carpros.com/articles/bad-catalytic-converter-symptoms

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

I would suggest to have the catalytic converter tested to see if its plugged or defective. Those temperature readings suggests it's plugged. Keep us updated and let us know how things turn out. Hope this helps and thanks for using 2CarPros.
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Sunday, October 11th, 2020 AT 10:25 PM
Tiny
MONDOFOX
  • MEMBER
  • 6 POSTS
Thanks Danny. I'll test again tomorrow. Seems odd that a brand new catalytic converter would be plugged.
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Sunday, October 11th, 2020 AT 10:53 PM
Tiny
DANNY L
  • MECHANIC
  • 5,648 POSTS
Hello again.

As I stated in last post that is the catalytic converter code. After having tested tomorrow let us know what they find and we'll go from there. Hope this helps and thanks again for using 2CarPros.

Danny-
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Monday, October 12th, 2020 AT 12:33 AM
Tiny
MONDOFOX
  • MEMBER
  • 6 POSTS
200-mile old new catalytic converter once again failed the temperature test today. 550 inlet 400 outlet at idle. 680 inlet 460 outlet at 2,500 rpm's. Vacuum test good. O2 upstream bouncing voltages at all rpm. O2 downstream consistent at.1 at 2500 rpm, fairly consistent at.7 with an occasional drop at at idle.
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Monday, October 12th, 2020 AT 5:32 PM
Tiny
DANNY L
  • MECHANIC
  • 5,648 POSTS
Hello again.

Was the upstream 'bouncing' o2 oxygen sensor replaced after the repair? Also, When you had the catalytic converter did they test and see if your muffler was plugged and causing too much upstream back pressure? Sorry you are having continued issues on this but we will get it figured out. Let us know and we'll go from there. Hope this helps and thanks again for using 2CarPros.

Danny-
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Monday, October 12th, 2020 AT 8:58 PM
Tiny
MONDOFOX
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  • 6 POSTS
No, the upstream sensor is several years old. The downstream is original. With the exception of occasional fluctuation in the downstream sensor, both are acting about like you'd expect. There are no back pressure issues. No misfires on cylinder 2 and 3, only 1 on cylinder 1 and 3 on cylinder 4 - not enough to be a concern. The only obvious measured malfunction so far is the temperature into the catalytic converter being 150 degrees hotter than the output temperature.
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Monday, October 12th, 2020 AT 10:51 PM
Tiny
DANNY L
  • MECHANIC
  • 5,648 POSTS
Hello again.

Yes, having a misfire will generate more heat and damage catalytic converters. A combination of higher heat and raw fuel from a misfire will ruin catalytic converters. Are you having a misfire? Get back to us and we'll go from there. Thanks again for using 2CarPros.

Danny-
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Tuesday, October 13th, 2020 AT 12:49 PM
Tiny
MONDOFOX
  • MEMBER
  • 6 POSTS
No unacceptable misfires. None at all on three of the cylinders. Only 5 with 35,000 permissible EWMA on one cylinder.
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Tuesday, October 13th, 2020 AT 6:49 PM
Tiny
DANNY L
  • MECHANIC
  • 5,648 POSTS
Hello again.

Any type of misfire small or big will still send raw fuel through the exhaust and catalytic converter and ruin it. What does EWMA stand for? Are you going to swap out the converter again? Have you noticed if your car has been running hot lately or higher than normal? Let me know if you need any other information or resources. Hope this helps and thanks again for using 2CarPros.

Danny-
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Tuesday, October 13th, 2020 AT 8:16 PM

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