P0420 catalytic converter check engine light came on?

Tiny
BEETLEPUNK21
  • MEMBER
  • 2009 LEXUS RX 350
  • V6
  • 2WD
  • AUTOMATIC
  • 199,000 MILES
Greetings, you amazing mechanics!

The check engine light came on in the VSC light started blinking. My scanner showed P0420 code relating to the catalytic converter. After doing a little research, I saw that it could be multiple different reasons, and may not actually be the catalytic converter failing. This is where I defer to your amazing mechanic expertise! How would you go about diagnosing the issue?
Thank you in advance!
Thursday, January 8th, 2026 AT 11:02 AM

9 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,192 POSTS
Hi:

In most cases, a P0420 does indicate the converter is operating below threshold and needs to be replaced. There are different products that are available that are designed to clean and regenerate them, but they don't always work. From experience, Lucas has come out with a new product, and I have seen the best luck with it, but the directions need to be followed exactly.

As far as what you are dealing with, there are ways to determine if the converter is actually at fault. By using an infrared thermometer, you can check converter temps to determine if it is working. The temp should be hotter at the outlet.

Here is a link that explains how to check it. Take a look at this link and try it. If you have questions, please let us know.

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

Let me know if this helps.

Take care,

Joe
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Thursday, January 8th, 2026 AT 7:52 PM
Tiny
BEETLEPUNK21
  • MEMBER
  • 146 POSTS
Hey Joe, here are my results. The temp would fluctuate at the inlet and outlet of the cat, both higher and lower than the temps I was able to capture. What do you think? Is it shot? Or do you think it’s still good? Thank you!
Images (Click to make bigger)
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Tuesday, January 13th, 2026 AT 6:30 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 55,018 POSTS
Yep, thanks for the images, that catalytic converter seems no good, I would remove the exhaust manifold and inspect the honey come inside. It is probably melted inside. Let us know, Ken
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Wednesday, January 14th, 2026 AT 7:28 AM
Tiny
BEETLEPUNK21
  • MEMBER
  • 146 POSTS
Hello, thank you for the reply. I will try to work on removing the manifold to inspect the honeycomb. I am assuming if the front cat is no good, then the rear cat is probably no good, or at least close to the end of its life? And I should probably replace both?
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Wednesday, January 14th, 2026 AT 7:44 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 55,018 POSTS
I would replace just the front one, it takes all of the abuse. The rear cat plays clean up unless the front cat came completely apart and has plugged the rear cat, then you need to do both.
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Thursday, January 15th, 2026 AT 10:57 AM
Tiny
BEETLEPUNK21
  • MEMBER
  • 146 POSTS
Thanks, Ken. You rock!
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Thursday, January 15th, 2026 AT 2:35 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 55,018 POSTS
You are welcome :)
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Friday, January 16th, 2026 AT 7:57 AM
Tiny
BEETLEPUNK21
  • MEMBER
  • 146 POSTS
Hello, when I first made this post, I had cleared the code so that the check engine light isn’t on. It came on again yesterday and so I went to go clear it, and when I pulled up the code, it was P0420 of course but what I didn’t realize the first time is that it said bank one. The code said something like catalyst efficiency on Bank one below threshold. So I am writing to you guys to confirm that the rear side of the engine (2009 lexus rx350 v6) closest to the firewall is bank one, correct?

And if you scroll up in my previous messages, you’ll see that when I did the test on the catalytic converter at the front of the engine, that was bank two, correct?

Do you see where im getting at?
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Saturday, January 17th, 2026 AT 6:40 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 55,018 POSTS
Yep, I would check them both, but start with bank one which is near the fire wall.
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Saturday, January 17th, 2026 AT 9:39 AM

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