P0420-Catalyst system efficiency - Scion tC Forums
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#1 (permalink) Old 07-03-2008, 10:39 PM
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Hey all I have a CEL on, and this is the code that came up: P0420 - Catalyst System Efficiency Below Threshold Bank 1. I did some searching around and found a few people claiming its either the rear O2 sensor, the front O2 sensor (or A/F sensor??), or the catalytic converter. But on most forum posts I've seen, no one replied back to let people know if replacing em all was successful. Just knowing if anyone has run into this code and successfully fixed it so it didn't come back?

I just got the rear o2 sensor from the dealer and had my buddy mechanic put it in. He reset the comp, and 5 miles down the road the CEL came back on. So I guess next guess is the A/f sensor?

I am saving the catalytic converter for last because that's the one thing that is still under warranty on my car (70k miles). As far as I can tell in the scion manual, the cat is covered til 80k. So I figure if I can replace both the sensors for cheaper than dealer rates, when I bring it in they can't try those and charge me *crosses fingers*.

Anyway, if anyone has input on this... like what I should try, what I shouldn't bother trying I would really appreciate it!!

Oh - back story, last year the CEL came on a few times, but would go off after a few hundred miles, then stay off for like a month or so at a time. I always figured it was my gas cap. But now its been on for longer than normal and I need to get it inspected!!!!! AHHH!

Also-I have no aftermarket parts on it, its all the same car as when I bought it. Mechanically, anyway.
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#2 (permalink) Old 07-04-2008, 08:50 PM
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Go to the dealership and get the Cat replaced. I don't understand how a p0420 can lead to a bad a/f senser (first sensor). All this is monitoring are the 2 oxygen sensors. If the second one follows the same path as the first one (goes rich lean rich lean rich lean etc), either the o2 sensor is bad or the cat is bad. What a properly working system should look like is the first one fluctuates while the second one barely fluctuates. Wish I had a picture of an oscilloscope monitoring both sensors as it would make everything much easier to explain.

I also hope you used a OEM sensor as after market sensors never seem to agree with Toyota ECUs.

My vote is to take it to the dealership, let them know what you have done and have the diagnose it.

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#3 (permalink) Old 07-05-2008, 01:42 PM
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I assume the o2 sensor that I got is OEM -- I got it directly from the dealer. Good to know that the A/F sensor probably wouldn't be triggering that P0420. That saves me from trying that out. I think I'll just take your advice and drop it off at the dealer - it'll save me extra headache anyway. I will post back with updates.
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#4 (permalink) Old 07-09-2008, 11:23 PM
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QUOTE (silvertc106 @ Jul 5 2008, 08:42 AM)
Quote:
I assume the o2 sensor that I got is OEM -- I got it directly from the dealer. Good to know that the A/F sensor probably wouldn't be triggering that P0420. That saves me from trying that out. I think I'll just take your advice and drop it off at the dealer - it'll save me extra headache anyway. I will post back with updates.[/b]
I know it's soon - but im just wondering if you have any news...
I just got the same code after my CEL came on. It came on first time last week and I took it to the Autozone next to where I work. The guy there asked me if I knew to tighten my gas cap until it clicked... OBVIOUSLY I do. I drove it the last 40k miles since I bought it without that problem... Oh well. He wouldnt tell me the code # and just cleared it, turned my gas cap and said see if it comes back on. So of course it did, like I knew it would. I took it to a different Autozone today by my house and they told me p0420 was the code and that I needed to replace both o2 sensors. Of course you never know what you're getting at autozone (as far as the people).
Thats why I was checking here first to see if anyone else had something similar. This autozone guy didnt sound too sure of himself and he didnt know where to look for scion in his computer to price the o2 sensors. He acted like I was stupid when I told him to look for scion in the make section; he had to be about 70 YO though.
I noticed your car had around 70k on it - Mine has 67. If it is the cat I want to get that fixed at the dealership while I still can.
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#5 (permalink) Old 07-13-2008, 12:38 PM
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OK, as far as I know problem is now solved. I had replaced the rear O2 sensor with no luck, then after Eric's post I dropped it at the dealer. They replaced the catalytic converter under warranty, with no problems. After that they reset the computer so the light was back off. I drove the car for about 60 miles and it didnt come back on. So I took her over to Jiffy Lube for inspection attempt number 2, and it passed!!

Since inspection I have driven it another 60 or so miles (so 120ish miles since I last saw my CEL), with no problems. Car feels a bit more peppy too now So in my case it was actually the converter. Definitely drop your car at the dealer before that 80k mark if you have this code. That converter would have cost me around 500 bucks plus labor if it wasn't covered.

Hope this helps others who have that crappy code hehe.
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#6 (permalink) Old 07-13-2008, 08:06 PM
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QUOTE (silvertc106 @ Jul 13 2008, 07:38 AM)
Quote:
OK, as far as I know problem is now solved. I had replaced the rear O2 sensor with no luck, then after Eric's post I dropped it at the dealer. They replaced the catalytic converter under warranty, with no problems. After that they reset the computer so the light was back off. I drove the car for about 60 miles and it didnt come back on. So I took her over to Jiffy Lube for inspection attempt number 2, and it passed!!

Since inspection I have driven it another 60 or so miles (so 120ish miles since I last saw my CEL), with no problems. Car feels a bit more peppy too now So in my case it was actually the converter. Definitely drop your car at the dealer before that 80k mark if you have this code. That converter would have cost me around 500 bucks plus labor if it wasn't covered.

Hope this helps others who have that crappy code hehe.[/b]
An o2 sensor can throw that code if the seccond o2 sensor is reacting slowly. If it is not reacting quickly, it will lead the computer to think that the Cat is doing nothing, or not enough.

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#7 (permalink) Old 12-22-2008, 05:47 PM
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sorry for necro posting, but i didnt want to create new thread.
ive got a problem, that seems my o2 sensor is dead. so i wanna ask, if someone has tried to put NOT oem sensor as an aftermarket, as it is 6 times cheaper than oem. and ive got no warranty (my car has done 28k miles). i know oem is the best etc, but theres loads of money id save..
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#8 (permalink) Old 12-22-2008, 05:59 PM
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how do you not have warranty with 28k miles

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#9 (permalink) Old 12-25-2008, 07:23 AM
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QUOTE (sciontC'06 @ Dec 22 2008, 11:59 PM)
Quote:
how do you not have warranty with 28k miles[/b]
im in europe. lithuania.
if u were in mexico, theres no one to provide warranty services. the same is here, no scion dealerships.
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#10 (permalink) Old 12-25-2008, 02:10 PM
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Go with an OEM sensor, I know its expensive. But, in the long run its the best route to take.


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