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Discussion Starter #1
So with 250,000 miles I got the infamous P0420 code. I figured for 100 I’ll throw in another converter on it and then I ordered a sensor to go in the converter. My light still came on. Frustrated I researched and didn’t realize there was an upstream sensor. So I bought a brand new upstream sensor so now I had two new sensors and a new converter, and it still brought on the light. I finally brought it to a garage even though I have 50 years experience and he came back with the conclusion that my brand new converter is no good. So today I put the old converter back on and the light is gone. So this whole time it was the upstream sensor. eBay buyer will not give me my money back because my emission sticker says it’s for California cars and this is a non-California converter even though the other one that’s on it is now working fine it’s not a Ca converter either. Hope this information helps somebody. The garage told me for about 400 He could put on a heavy duty converter. Thinking it’s probably the California type one? And he said it’ll pass. But for now I’m good.
He did tell me the engine has solid lifters but I have a strong vacuum so he doesn’t think that that’s the problem and I think he did check my air mass sensor.
 

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Good to know to check both sensors before switching out catalytic converter. Thanks for the cautionary tale.
 

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Discussion Starter #4
So you're saying the sensor on the cat right in front of the radiator caused your issue?
Not the downstream sensor in the cat but the one under the engine, yes closer to the radiator. The upstream one. I never knew there was an upstream sensor so it had 250 K on it. I checked it again today and I have no codes. I’m so excited
 

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Discussion Starter #6
No the car ran great no miss firing etc. That’s what was driving me crazy. I didn’t know about the upstream sensor. So I knew I did the converter and I knew I did the only sensor I knew of so why was the stupid light still coming on. Then I take it to a garage and the guy tells me I have a bad cat. And I’m like what? It’s brand new. But he was right. I put on the old cat and the light went off so now I know it was that upstream sensor that I wasn’t aware of until I went on YouTube and saw it on a video
 

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Discussion Starter #8
133809
7FB24095-2D5E-4B05-9FFB-24CAE963AFB5.jpeg

Inquiring minds want to know, including mine.
So here’s the thing they tell me that I bought the wrong converter because my emission sticker says California emissions. But this is the exact same type and size of the one I bought five years ago which was not a California converter that it’s back on the car working here in Ct
 

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I think it's the upstream sensor we wanted to know about. Isn't that what solved your problem?
 

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OK great, so what part number and brand upstream sensor did you get?
 

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Discussion Starter #14
Would’ve never gotten it off without this tool. The one in the cat was almost welded in the one under the engine came off easy
133812
 

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I'm cheap. But I'm not "get your cat for $89 on Ebay" cheap. Personally I'd pay $200 and get a Walker branded cat from RockAuto.... if I needed one. Glad you got your issue fixed. Lots of people say if there is a problem with the upstream sensor, you won't throw P0420, you will throw some other code. Looks like that wasn't the case for you.

Cats:
Sensors:

Either way, cat or sensors, these are impressively inexpensive.
 

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Lots of people say if there is a problem with the upstream sensor, you won't throw P0420, you will throw some other code.
Here's my O2 sensor adventure. Code was P2254, which I've never seen anywhere else.
 
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