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Old 01-15-2012, 08:42 PM   #406 (permalink)
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Originally Posted by augustine13 View Post
I just got done talking to a buddy of mine who is a mechanic, and he said that it wouldnt throw the code because the all the rear sensor does is make sure the cats are functioning.
The rear O2 sensor monitors the efficiency of the CAT(s) by monitoring the exhaust flow for excess O2. That excess O2 can be caused by failing CAT(s), or CAT(s) being overwhelmed by an overly rich Air/Fuel mixture & of course a leak in the exhaust system allowing O2 into the exhaust flow. When The rear O2 sensor detects excess O2, it reports that to the ECM, which will set the P0420 CEL code. Of course the rear O2 sensor could be bad, but that should give you a different CEL code.

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So the only things that could be causing the code is a bad rear sensor, bad cats or a leak between the sensors.
I agree, but don't limit your trouble shooting to just that, since the problem could be much farther up stream?

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Old 01-15-2012, 11:03 PM   #407 (permalink)
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I agree. From what I understand it could be many different things. I have a friend who can get me the sensors at cost, so im going to start there. Theres also a good chance that I could have a bad fuel pressure sensor as well(not sure if that could be causing the p0420 problem) still plan on replacing it though. What about just taking out the cats and straight piping it? while spacing out the rear sensor.
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Old 01-16-2012, 09:05 AM   #408 (permalink)
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Some questions:
  • How's your gas mileage. Does it seem normal, or does it seem low?
  • How's the engine performance. Does it feel normal or lacking power?
  • Any hesitation or back fires?
  • How's the exhaust tip look. Is it overly sooty?
If you have any of the above problems & you're thinking about replacing the O2 sensors, I'd suggest starting with the front O2 sensor.

For the front O2 sensor, I always recommend going with a factory/OEM (yes, expensive). That sensor is critical for correct engine operation. You can go cheap on the rear O2.

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Old 03-07-2012, 03:53 AM   #409 (permalink)
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I just tackled this code, too. After reading through this sticky, I decided to start with replacing the downstream O2 sensor. While doing that, I found two tiny holes in the flange right before the downstream converter. Patched those up, installed sensor, and cleared the code. Car passed inspection, and the light is still out.
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Old 03-07-2012, 11:46 AM   #410 (permalink)
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I just finished reading the entire 28 pages of this thread and I would like to thank you all for contributing to it, I have found it quite thoroughly fascinating and informative.

My 2003 Forester XS with 100k miles is getting this code. It first came on about a month to month and a half ago. At first she ran fine, but she has since developed the following symptoms:

- Soot from the tailpipe. It even drips at times.
- Decreased gas mileage. I was getting 25/mpg and it has gradually decreased to about 22-21/mpg
- I haven't noticed any hesitation, but I have noticed it idling a bit rough, though it is slight enough that I might just be going crazy

I am thinking about starting with Sea Foam in the vacuum line and the oil and Techron Engine Cleaner in the gas, in the hopes that they clean something that is dirty, I figure even if doing that doesn't help my current situation at the very least it should clean things up a bit.

After that my next route would be replacing the upstream sensor. I did get a code for that sensor several months ago and I took it in. They just told me it was a loose connection and that was that, but perhaps it is actually going bad?

Open to suggestions and/or comments. Thanks again!
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Old 03-07-2012, 02:14 PM   #411 (permalink)
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I see this is your first forum post, so welcome to the forum from Oregon!

My first thought is a failing front O2 sensor, especially if it hasn't been changed. What CEL code(s) are you getting?

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Old 03-08-2012, 05:36 AM   #412 (permalink)
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Why, thank you for the welcome. I have been lurking on this forum for some time now, it is very helpful. =)

The only code I'm getting is the P0420.

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Old 03-08-2012, 08:59 AM   #413 (permalink)
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Many things can cause the P0420 CEL code. It only takes a small hole in the exhaust, ahead of the CAT, to give you this CEL code.

On your '03 Forester, you could try the $5 fix, but keep in mind that's really not a "fix", more correctly, a work around.

As I wrote on the other post, if your front O2 sensor hasn't been replaced, it may be failing. When they start to fail, they'll richen up the Air/Fuel mixture. That can cause:
  • Poor gas mileage.
  • Poor performance.
  • Hesitation.
  • Overwhelm the CAT & the rear O2 sensor will give you a P0420 CEL code.
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Old 03-08-2012, 10:50 AM   #414 (permalink)
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I did take a look at the exhaust and I was unable to make out any holes.

As for the $5 fix, it is kind of funny cause I was in fact driving home from the emissions test when the light first came on. The light itself being on doesn't really bother me, I'm more concerned with fixing my gas mileage. I drive >50 miles a day, so it adds up pretty quickly.

I will order a new front o2 sensor, since it hasn't been replaced. As per your recommendation it will be factory/OEM. I need to do the front brakes anyway, so I'll just do it all at once. Hopefully that'll be the end of it.
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Old 03-08-2012, 10:57 AM   #415 (permalink)
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The low gas mileage could be related to winter gas, if your area uses a 10% ethanol mix during the winter. We have E10 year round herer in Oregon.

The $5 fix won't fix your low gas mileage. I'm going to guess you're running overly rich & the new front O2 sensor will correct both the poor gas mileage & the P0420 CEL code.

If it were me, I'd go with a factory/OEM front O2 sensor, but members have had good luck with Denso direct fit front O2 sensor.

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Old 03-18-2012, 08:11 AM   #416 (permalink)
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Since my check engine light went out after an emission test(CA), it went ON almost like clockwork every 500 mi . Did nothing before or after to the system and have 10K on the clock after the test and no light ever again. Another person had the light right AFTER the test. Could it be in the ECM, making the parameters a little too tight? I know that the state test should be a read only kind of test. Just throwing some facts out there, I don't make the connection how the tests can affect our systems!!!!
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Old 05-02-2012, 08:29 PM   #417 (permalink)
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Where exactly did you find these leaks and how did you know they were there. I am wondering if my pesky code is being triggered by a pinhole leak, too, since I can detect no major exhaust leak that I can find. Or, of course, there is something else causing it.


Quote:
Originally Posted by Wrench01 View Post
I just tackled this code, too. After reading through this sticky, I decided to start with replacing the downstream O2 sensor. While doing that, I found two tiny holes in the flange right before the downstream converter. Patched those up, installed sensor, and cleared the code. Car passed inspection, and the light is still out.
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Old 05-03-2012, 12:07 AM   #418 (permalink)
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Here in California there are very strict regulations when replacing a catalytic converter. Seems every time an old car fails the smog test the repair shop puts in a new converter. Of course the car then passes but very soon - about 2 months, if the car is tested again for some reason, it will fail again. The converter maybe bad but the reason why it went bad has to be fixed or the new one will fail too. Something to think about.

An approved test, which must be documented by a smog test center before replacing a converter, is called the converter crank test. What the technician does is put the emissions measuring probe in the tail pipe of a warmed up converter. He disables the ignition system and cranks the engine with the throttle wide open. The fuel/air mixture passes into the converter. If the probe measures 12 percent CO2 the converter is good. Why? Because only a fully functioning converter will take HC and O2 and convert it or burn it to produce CO2!

There are many smog centers that will run this test for you for a small fee.
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Old 05-25-2012, 12:28 AM   #419 (permalink)
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anybody tried this method? lol

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Old 05-25-2012, 07:58 AM   #420 (permalink)
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I have an old Datsun 240Z. Straight line 6 with 2 carburetors. Does not even have catalytic converters. Does not even have to be smog tested! I took it down to the smog tester to adjust my carburetors. I got the mixture adjustments perfect. More power, more MPG, and less pollution!
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