P0420 - Catalytic Converter Efficiency Below Threshold - Page 5 - Subaru Forester Owners Forum
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post #61 of 770 (permalink) Old 01-19-2009, 01:49 PM
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when you have a P0420 code... does it go away once its back within threshold? or do you have to reset it and hope it doesn't return? :-)


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post #62 of 770 (permalink) Old 01-19-2009, 03:46 PM
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excellent post ddavidv - many thanks

I've tried looking for one of those spark plug non fouler things here in the uk with no joy - any ideas guys?
PM me and I'll see if we can work out an arrangement. Swap you for some issues of MINI magazine or something (I have a '79 850 saloon).


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post #63 of 770 (permalink) Old 01-21-2009, 08:32 AM
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Originally Posted by Torsen View Post
when you have a P0420 code... does it go away once its back within threshold? or do you have to reset it and hope it doesn't return? :-)
I believe I'm dealing with a dying cat in my 2001S.
That code comes on at speed, in my experience.
A week of slow, stop-go trips and the check engine light goes out.
Today I hopped on a throughway and as soon as I topped 55 mph, the light came back on.
Question: Is this indeed indicative of a dying cat?

Last edited by jpspecial5; 01-21-2009 at 08:37 AM. Reason: question added
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post #64 of 770 (permalink) Old 01-21-2009, 09:06 AM
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I always get this on the highway as I`m prepared to exit. I`ll turn off cruise, be off the gas, then look at dash and CEL is flashing. Its happened once while in cruise.

Took it to dealership, they ordered a new cat, just need to go get it installed.
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post #65 of 770 (permalink) Old 01-21-2009, 10:43 AM
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Originally Posted by jpspecial5 View Post
I believe I'm dealing with a dying cat in my 2001S.
That code comes on at speed, in my experience.
A week of slow, stop-go trips and the check engine light goes out.
Today I hopped on a throughway and as soon as I topped 55 mph, the light came back on.
Question: Is this indeed indicative of a dying cat?
We have a different issue on my son's '03 X,the CAT is overwhelmed on full boost!

If your engine is running normally, no sooty exhaust tip or funky smell, it could be the CAT, or a defective rear O2 sensor. If you have a sooty exhaust, it could mean the front O2 sensor is failing (does before a code is dropped), & it's causing the engine to run too rich. Of course there could be other issues farther upstream.

The least costly fix is the mechanical one using the modified spark plug non-fouler. This moves the sensor out of the exhaust flow, so it isn't as sensitive. I'd start with one & add the second if you need it.

Here's a couple more threads to peruse:

new Cats and O2 sensors
cat converter for my 2000 forester

Bobby...

['07 FSXT MODding Journal] ['03 X MODding Journal]

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post #66 of 770 (permalink) Old 01-21-2009, 12:48 PM
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Thumbs up

Thanks, bobby
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post #67 of 770 (permalink) Old 01-21-2009, 06:52 PM
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No problem, glad I was of help!

Bobby...

['07 FSXT MODding Journal] ['03 X MODding Journal]

'07 FSXT - COBB Surgeline dyno stock Protune - Stage 1+
'03 X +AVO turbo kit = XT (son's)
'95 Neon Sport

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post #68 of 770 (permalink) Old 01-22-2009, 03:27 AM
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Quote:
Originally Posted by 2.5x_sleeper View Post
If your engine is running normally, no sooty exhaust tip or funky smell, it could be the CAT, or a defective rear O2 sensor. If you have a sooty exhaust, it could mean the front O2 sensor is failing (does before a code is dropped), & it's causing the engine to run too rich. Of course there could be other issues farther upstream.

Bobby...
Just to follow up, does a dead cat have a smell of its own?

(The engine runs fine. I do have a bit of sooty deposit around the exhaust tip, but the exhaust gas itself seems clean. and no funky smell of any kind is being emitted from anywhere.)
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post #69 of 770 (permalink) Old 01-22-2009, 08:32 AM
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If CAT is bad or overwhelmed, it can smell like rotten eggs. Our engines tend to be on the rich side, so some soot on the exhaust tip is normal.

Bobby...

['07 FSXT MODding Journal] ['03 X MODding Journal]

'07 FSXT - COBB Surgeline dyno stock Protune - Stage 1+
'03 X +AVO turbo kit = XT (son's)
'95 Neon Sport

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post #70 of 770 (permalink) Old 01-28-2009, 06:45 AM
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Cheap fixes won't work here in PA where they go by the OBDII for emissions testing - all sensors have to show ok, and I believe the $5 mechanical fix won't help there. I've had the P0420 code for a few weeks now on my '01S. It cleared a few days before the 48hrs of Tri-State run when I installed a new battery, so I let it go; it came back on about 1,000 miles later, and won't clear for more than a few miles now.

Going by the old method of eliminating the causes one at a time, starting with the cheapest, I just received an Eastern cat (universal fit, but with the 60-degree bend) for $75 online, which I will get the local Midas shop to install tomorrow after work. Then, I'll clear the code and drive as much as I can that night to see if it comes back, and if the car will then pass OBDII state emissions inspection. If it comes back, or if my reader still shows it won't pass emissions, I'll swap out the rear O2 sensor.

Any better ideas, or am I following the best path?


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post #71 of 770 (permalink) Old 01-28-2009, 06:47 AM
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Originally Posted by KeNd0 View Post
I always get this on the highway as I`m prepared to exit. I`ll turn off cruise, be off the gas, then look at dash and CEL is flashing. Its happened once while in cruise.

Took it to dealership, they ordered a new cat, just need to go get it installed.
I don't believe the CEL will flash with just the P0420 code. Emissions-related codes are a steady light; the blinking CEL indicates a more serious code.

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post #72 of 770 (permalink) Old 01-28-2009, 06:55 AM
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The mechanical fix does clear the fault, so the affected sensor (rear O2) will show normal function. If your State test just connects to the OBDII port, it will pass. If they put a sensor up the tail pipe, it may, or may not pass, depending on when your sensor was was failing. Mine was only failing under extreme acceleration.

As to replacing the parts to correct the fault. For you, the CAT may cost less, mine was around $125 to have a high flow CAT installed by my trusted muffler shop. If you're looking easier, I'd do the rear O2 sensor. Another thought is a failing front O2 sensor causing your engine to run too rich, overwhelming the CAT & being detected by the rear O2 sensor (monitors the CAT).

Bobby...

['07 FSXT MODding Journal] ['03 X MODding Journal]

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post #73 of 770 (permalink) Old 01-28-2009, 06:58 AM
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I'm also trying to toubleshoot this in the most efficient manner, as well as not spending more than i have to, as my state inspection is up on Saturday!

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post #74 of 770 (permalink) Old 01-28-2009, 07:28 AM
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i purchased my forester out of state so i had to get it inspected by the sheriff, i gave him the keys... he walked around the vechile and handed me the ok form... LOL :-D

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post #75 of 770 (permalink) Old 01-28-2009, 08:01 AM
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if any xt in socal/orange county needs a cat because yours is failing, i have one for free. pm me

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