Cat issue diagnosed it with a P0420 code?

Tiny
ZAFEWORK
  • MEMBER
  • 2011 LEXUS IS 250
  • 2.5L
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 92,000 MILES
I bought a Lexus IS250 a few months ago, and right after driving it off the lot, the check engine light came on. I took it back to the dealership, and they diagnosed it with a P0420 code. They replaced the catalytic converter, got it to pass emissions, and everything seemed fine. A few weeks later, the check engine light came back on with the same code. I was confused, but then it went away, so I figured it might’ve just been a glitch. After a long drive, the light came back on. I took it to a mechanic, and he said it wasn’t the catalytic converter or the O2 sensor, but he couldn’t pinpoint the issue. He cleared the code and said it might’ve been a false reading. The light stayed off for about a month, but after another long drive, it came on again. I’m not sure what’s going on, any help is appreciated!
Thursday, June 26th, 2025 AT 11:11 AM

1 Reply

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,591 POSTS
So this code is straight forward, the catalytic converter is not doing it job, or there is an exhaust leak in the system, or the engine is getting too much fuel. Do you hear an exhaust leak while accelerating?

P0420 - Catalyst System Efficiency Below Threshold (Bank 1)

Also, you can remove the spark plugs and look for one that is black, this will be the cylinder with the leaky fuel injector. Also, if you scan the car please tell me the long and short erm trims and I can tell you for sure if the cat is bad.

This engine uses a dual manifold + cat setup, commonly called manifold cats or pre-cats. So which ones did they replace, typically the exhaust manifold cats good bad first. I would look and try to confirm the replacements, make sure there are not just spray painted or some other tomfoolery. Please upload pictures or videos in your response so I can see?
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Thursday, June 26th, 2025 AT 5:11 PM

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