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Code PO420, Emission System?

Tiny
BADBOYROD
  • MEMBER
  • 2020 GMC TERRAIN
  • 1.5L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 124,000 MILES
PO420 code, Check engine light came on a week ago. I put a bottle of Cataclean and seemed to help. No check engine light for about a week. I got off work today and Check engine light is back on. I was told it could be catalytic converter.
Wednesday, July 17th, 2024 AT 1:32 PM

11 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,747 POSTS
Hi,

If the same code returned (P0420), there is a good chance that is the problem. The code indicates the catalytic converter is operating below expectations. Often, the converter is faulty or simply worn out.

An easy thing to do is this. Get your hands on an infrared thermometer. Point it at the inlet of the converter and note the temperature. Then, do the same at the outlet of the converter. The outlet should be hotter, 30 to 100 degrees higher. If that is found, the converter isn't the problem. If you find the temperatures are near the same, or the outlet is cooler, the converter is faulty. Take a look at this link. It better explains what I mentioned and provides more information on testing.

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

Try this and let me know what you find. Also, if you have other questions, let me know that as well.

Take care,

Joe
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Wednesday, July 17th, 2024 AT 8:08 PM
Tiny
BADBOYROD
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  • 16 POSTS
Hello,

If the converter is faulty, can you send me a video or diagram on how to replace it? Iam going to replace it myself. Thank you!
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Thursday, July 18th, 2024 AT 5:00 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,747 POSTS
Hi,

No problem whatsoever. Note that one end bolts to the exhaust manifold and the opposite end bolts to the exhaust system. If rust is a concern where you live, spray the bolts with some penetrating fluid in advance to help.

If you look below, I attached the directions from my manual. There is a bracket that will need removed.

Let me know if this helps or if you have other questions.

Take care,

Joe

See pics below. Note, make sure the engine is cool before you do this procedure. Also, it is a good idea to disconnect the negative battery cable.
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Thursday, July 18th, 2024 AT 7:24 PM
Tiny
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  • 16 POSTS
Thank you for y’all help. I replaced catalytic converter and that was the issue. Running better.
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Monday, August 19th, 2024 AT 8:08 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,747 POSTS
Hi,

Thanks for the update. I'm glad you got it resolved and it's running better.

Take care of yourself and feel free to come back anytime in the future.

Joe
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Monday, August 19th, 2024 AT 7:16 PM
Tiny
BADBOYROD
  • MEMBER
  • 16 POSTS
Hello,
I replaced with a new catalytic converter, few days after I get a rattling noise every time at startup but goes away, check engine light is back on, PO420 Catalyst System efficiency below threshold bank 1. What can it be? Please help.
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Saturday, August 24th, 2024 AT 11:41 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,747 POSTS
Hi,

The rattle is interesting. Do me a favor, with the engine cold, take a rubber mallet and tap on the exhaust system to see if you can recreate the sound.

If you can, check to see if it is coming from the new converter.

Let me know.

Joe
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Saturday, August 24th, 2024 AT 7:31 PM
Tiny
BADBOYROD
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  • 16 POSTS
Got it figured out it was the clamp that connects between the turbo and catalytic converter, it was loose I didn’t tighten it enough. I just didn’t want to over tighten it. No rattling noise. Thank you for your help.
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Tuesday, August 27th, 2024 AT 4:50 AM
Tiny
BADBOYROD
  • MEMBER
  • 16 POSTS
I replaced the catalytic converter with a new one and replaced 02sensors new, and check engine light is back on again this is the 3rd time, everything is tightened nothing loose gaskets are on. I am out of options.
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Tuesday, August 27th, 2024 AT 4:54 AM
Tiny
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It’s giving me the same code PO420 catalyst system efficiency below threshold bank 1.
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Tuesday, August 27th, 2024 AT 4:55 AM
Tiny
JACOBANDNICKOLAS
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Hi,

Do me a favor. Take a look at the description and diagnostics to check below. See if anything there was missed.

Take care,

Joe

See pics below.
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Tuesday, August 27th, 2024 AT 6:45 PM

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