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The Lexus IS is a compact executive car sold by Lexus since 1999. The IS was originally sold under the Toyota Altezza nameplate in Japan from 1998.

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Replace Catalytic converter and P0420 still there

Hello there.

I own a 07 Lexus IS 250 AWD with 90k miles that replaced both bank 1 O2 sensors and also new cat converter but the ecm P0420 came back at 30 miles after driving.

I try this: 1. I disconnect the air fuel sensor before cat and drove the car and just one code came up P0051 (Why just this one and not the P0420)

  1. Short term fuel trim on bank 1 is -2.3%
  2. And long term fuel trim read -23.4%

I will appreciate all your help…

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These newer cars are super sensitive about the performance of the cats, and O2 sensors. Did you use OEM Toyota cats and sensors, or aftermarket ones?

If you didn’t use the Toyota OEM parts, then it’s usually due to the replacements not playing nice with the car. You really need OEM sensors and cats on the newer cars, sadly. It was fine 10+ years ago when the computers weren’t as touchy, but not after the EPA (and governments around the world) got picky about how forgiving some of these ECUs were. For really old cars (think 90s-early 2000s) it isn't an issue but these new cars will never work right with aftermarket sensors*.

*If you’re in a no-inspection state, just accept the P0420 code if an OEM sensor does not help and the cats are bad ;). We do it a lot in these states anyway after 10+ years because of how expensive it is to TS the system. It's normal. Unethical? Probably. Illegal? Heh, nobody pays attention.

This car is one of those new-fangled whiny ones, so OEM is a must. If you're lucky, it may be early enough to not be overly sensitive but unlikely. That said I suspect it is sensitive because it is a Lexus and Toyota is known to be fickle about those things on the Lexus side.

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Try cleaning the MAF (mass airflow sensor) and/or install a cat o2 extender module that places the downstream o2 sensors farther away from the cats themselves.

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Alex Garcia sera éternellement reconnaissant.
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