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Q: Downstream O2 sensor or catalytic converter? on 2002 Toyota Tacoma

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I had a check engine light for some time, and found it to be the upstream O2 sensor (P1135). I left it go for a while due to money issues, but just replaced it the other week. The check engine light cleared right away, but about a day later, returned, and now when I had it checked at AutoZone, I am getting a P0420 code. Is one of my cats now bad due to neglecting to change the upstream sensor, or could it be because of the downstream sensor going bad? I also was told that due to the increased amount of ethanol now being used in gas, that many people are getting false codes on their engines. Thanks for any help, I'm trying to replace anything myself, but don't want to spend the money on a part that isn't bad!
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First, the P0420 code is not likely to be caused by a failing rear O2 sensor, the code occurs when the rear sensor is working and reading parallel to the front sensor, indicating that the catalyst is not working up to spec.
It's possible that the catalyst was damaged by a failing front sensor causing the computer to add too much fuel in the mixture, causing the cat to overheat or lose capacity. It could also have lived it's life and failed due to natural causes. but I'd suspect the front O2 sensor being faulty to have contributed to the catalyst failure. Before I'd replace a cat, I'd carefully read the data from the front and rear O2 sensors to be sure I think that the cat's bad.
Toyota catalytic converters bolt in so replacing it's not too tough. Only thing, Toyota replacement parts are probably fairly expensive, so I recommend diagnosing before spending money.
If you want to find a specialist, here's a directory link for you:
SuperBob, thanks for the information. Before I posted this today, I cleared the Check Engine light, & it hasn't come on-with only a short ride home. I'm suspecting it will probably come back, but I will try & take some readings off the sensors or take to a mechanic if I have to. I have been looking online for replacement cats, as I'm sure I can change one myself without difficulty. I have found ones that aren't TOO expensive & listed to OEM specs. My next question would be WHICH cat has failed, if this is the case. There is the one off the manifold, which is prior to the upstream O2 sensor, & the one after, which I'm thinking would be the one to fail. Thanks so much again for your input-much appreciated!
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