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Hi, guys

Getting the bank 1 P0420 code again on my 2004 Tundra.

2 years ago (27k miles) I got the code (Catalyst system efficiency threshold below threshold bank 1) and changed my 02 sensor to a Denso 234-4161 and the code went away until today.

The truck had 178k in it and now has 205k miles on it.

Can the sensor be bad already? Maybe it’s just the gasket.

Anyway I got out my code reader.
I hooked up my scan tool and ran it. I checked other screens to see readings, even though I don’t know what those readings mean, if there good or not.
Maybe I should just buy the Dorman 47021 oxygen sensor gasket and changed it out. Not expensive to try.

I attached pics.

Thanks!
E76E73EF-E722-4098-A62F-281D32C4DBA0.jpeg CE51F5DC-C084-46E4-9B7A-79AB1A719EF3.jpeg D8BFD16F-2B00-44C6-8A92-85FF9B6D8D06.jpeg 211DC9AF-6543-42EE-A40E-63E81FDF7775.jpeg 1F91A1B4-DFB5-4157-9997-E938BD04ECDD.jpeg 2D8A78C4-25DD-4C12-A5BD-BFB78E8133EE.jpeg C3E4B5D7-F5FA-404C-8A90-E1778EB6AA88.jpeg F518F09B-E26C-4CE1-8701-6B802F9D0631.jpeg D1BBA58E-BC74-4FFC-8D59-DDD039D4EB26.jpeg
 

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The only way to properly troubleshoot 02 sensors is with a much better tool so you can watch the upstream and downstream sensors while you perform active fuel control to go rich and lean while watching the sensor waveform responses.
First, did you replace an upstream A/F 02 sensor or the after cat 02 Sensor?

Regardless, the first thing to always check for an fix are exhaust leaks anywhere from the rear sensor all the way to the engine and fix those first. If you are sure there are no leaks or still get the codes after fixing them it is time to replace the upstream sensors as a pair. To me they are pretty much a tuneup item above 150k miles anyway. You could try to use some 02 or catalyst cleaner for replacing the upstream sensors. If you replaced the upstream 2 years ago just replace the rear on that side.

Important, DO NOT, I repeat, DO NOT use any 02 sensors other than Denso on a Toyota!
 
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