cdilmore

Less than 2 years ago, I changed out the exhaust - from the y-pipe to muffler. And the O2 & knock sensors, PCV valve. About a month ago, P0420 returned. So I did the $5 fix. But to no avail.

Yesterday, I poured in a bottle of CRC Guaranteed To Pass - $8.50 at WalMart. 5 miles later, the CEL turned off. I thought it was sheer coincidence, until I started to make every green light. People have asked if I've been working out. Why my smile is whiter. Who cut my hair.

I gotta tell ya, I'm riding a little high right now.

Blue Fox

OH MAN! I have the code thrown up too, and it comes back every other day or so after I clear the code. Guess this means a new downpipe for me, no?

__________________
2004 XT - RIP
2005 XT - 5MT - JOURNAL

trailangel

I did read up on this.
They seem to have taken the ZDDP out a few years ago.
But the ethanol gets in the oil and mixes with phosphates and burns out into the exhaust and ruins the cats. So this could be the problem, especially for us CA drivers. Very few ethanol free stations in CA.
It was my first gut instinct when I had this problem, bad gas.
Maybe we should call CA driving severe and change the oil at 3750 miles to combat this P0420 cat code... wait.... that's what I did for the first 38,000 miles, I changed oil every 3750. After that I'm doing 5k mile change.
Maybe that's why they have the 11 year or 150k mile warranty on emission parts in CA, because they know the gas is bad here?

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TrailAngel
2010 2.5X 5MT

pleiad7

Yep, it's the phosphorous compounds that are created when the ethanol mixes with oil and gets recirculated through the PCV system. I'm wondering if installing an oil catch can or air-oil-separator would make an appreciable difference. I used to have a catch can, but uninstalled it before ethanol caught on.

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-Silke
04 FXT PP PSM ~ "Subed"
98 Forester S ~ "Slynki"

subnut

I have an 03 Forster with 135k mi on it. I had this code about a yr and a half ago. After changing the front O2 sensor it still had the code. I then searched around for the cheapest priced cat converter I could find and installed that. Well it worked fine for 1 yr and a half and the cade is back again. I checked my exhaust pipes and they look good without any obvious holes. I will check again with the engine running on the lift to make sure but this is where its at right now. Will advise later. I personally think Subaru has a technical problem with this as it is all over the internet and that is not good.

Duderotomy

^try the $5 mechanical fix.
I ended up doing this on my '02 Forester. I believe the "fix" works up to MY05.

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07 XT - VF43'd
02 S
03 Outback H6
98 S - H6/Ver.8 swap
90 Nissan Skyline GTS-t

Inspector

It's not limited to subarus, believe me. This just says that you have an underlying issue that is slowly decreasing the efficiency of the cat. An aftermarket cat will show a code in less time because they usually have less catalyst in them. Could be burning oil, running rich, etc. You will have to do more than replace the cat and O2 sensors if you want to actually fix the problem.

2.5x_sleeper

Assuming your engine is running normally & your gas mileage hasn't dropped, the cheapest fix would be the $5 fix.

If you have to pass emission testing, the $5 fix may be iffy?

We have the $5 fix on my son's '03 X, since it can run rich on WOT when the 5th injector kicks in.

Bobby...

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

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

rearviewmirror

Thank you, I do hope the part numbers for the 2006 Forester are the same!

It's about $600 worth of parts if buying online, I just might do this if I can find a shop in Australia to fit them. The cat is double that over here from a shop, and there aren't any discount OEM parts distributors that I'm aware of.

trailangel

I think there is truth in this.
After getting my front cat replaced, I drove 2500 miles.. no problems, and then out of the blue... misfires with codes again for cylinder 2 and 4, just like before, right before I threw the P0402 code. At the dealer they could not reproduce these misfire codes, and they drove my car over 50 miles trying to get it to misfire.
I shut the car off.... waited a minute and restarted.. no misfires .. codes still on.
Went home and the next morning replaced the spark plug wires. at 56,000 miles. Shouldn't have to do this, but maybe one went bad. No misfires for 2500 miles and then I get one.. and the car is so sensitive .. it will damage the cat....??? After 4-5 restarts CEL goes off.
So far car runs excellent.. good milage.
I've had other cars that misfired but didn't ruin the cat. convertor in the process while it was being diagnosed and fixed.
This Subaru is too tightly wrapped!

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TrailAngel
2010 2.5X 5MT

sequoia14

It says right in the manual that misfires can damage the catalyst. If you think about how close the cat is and how low it is on the engine, it is very likely for it to get damaged in a misfire event. There may also be other issues with the material of the cat, but I really think it is these misfire events that are spelling doom to some of these converters.

forester25xs

I have had this code (P0420) since i got my car three years ago and i kid you not, a friendly mechanic recommended this, a fuel additive called BG 44K, poured it in the gas tank, and have not had a cat or exhaust problem in a year. (after a year it came back). Theoretically, the chemicals burn and coat the catalytic converted and act as a cleaning agent, burning it hotter than normal. I am no expert about this, but this worked for me!

SixStarPugilist

Cats don't wear out. They fail when they are burning off higher levels of HC and CO than the car was designed to produce (rich mixtures, ignition misfires...).
Roger

Igwin

So Borts, can i assume that a higher octane fuel makes a difference and lessens the chance of codes getting thrown? As for me I just had a PO042 - evaporator emission system leak getting thrown, the mechanic said its nothing to worry about, "all too common on these old Subarus" as he put it, and if it keeps coming back I'll try the $5 mech fix. Otherwise it runs fine, so far.

pleiad7

The only difference higher octane fuel will make is in the weight of your wallet. Higher octane = more resistance against pre-detonation, which is an issue for turbo cars, but not the N/A Subies.

__________________
-Silke
04 FXT PP PSM ~ "Subed"
98 Forester S ~ "Slynki"