P0420 - Catalytic Converter Efficiency Below Threshold - Page 51 - Subaru Forester Owners Forum
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post #751 of 774 (permalink) Old 09-29-2016, 03:33 PM
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Quote:
Originally Posted by sdunlimited View Post
I can't recall if if I read it in a P0420 thread or not (maybe P0440?), but the fuel filler neck on the older Foresters has a bad habit of rusting out and throwing a code.
I went through the same process you did on our 2002 Forester (stock headers and cats w/200k miles, Stromung midpipe, STI axle back) and the drilled out anti-fouler on the rear cat worked. We'll still occasionally throw a code, and I can't recall right now if it's P0420 or P0440, but I'm 99% positive it's due to the fuel filler neck.

The worst for us was chasing an erratic misfire code. It went on forever much like you're experiencing here. After doing everything under the sun, it cleared up once we replaced the battery... :)
Thanks for the insight, I believe the fuel filler neck would trigger P0440 for evap related issues. What I never thought about was the relationship between the evap/emission system and the exhaust and how the two interact if ever?


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post #752 of 774 (permalink) Old 10-18-2016, 07:40 AM
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The fight continues, but I believe I am coming to a solution. I decided the other day to test my old O2 sensor(Denso) to see if it actually was broken and compare that to my newer O2 sensor(Bosch). The two O2 sensors are very different in size(both M18 thread) and function.Bosch was much larger and had a filter piece which resembled a cylinder with 3 long uniform cuts that ran length wise up the filter. The Denso was smaller with a filter piece that resembled a cylinder with uniform circular holes cut out of the sides. I used a propane torch to heat up both sensors while they were hooked up to my multimeter. The Denso took far less heat/time to show a reading and would cycle from 13m ohms down to 1m ohms and back at relatively steady and quick pace, while the Bosch took a lot of heat/time to show a reading and covered a cycle from 18m ohms to 1m ohms at a more uneven/erratic pace. The heater circuit on the Denso read 3.7 ohms while the Bosch read 5.8. I rewired the Denso sensor in for fun just to see if the computer would trip a code. I again received p0420. I then bought a new Denso front O2 sensor and switched out the Bosch I had in place. The Denso and Bosch O2 sensors looked identical and I did not do any multimeter tests. After running the Denso(front/rear) for about 400 miles, I now have P0136. The only difference I can tell between the 2 front O2 sensors and the cars operation is that on initial start up the car revs lower and returns to normal idle rev faster with the new Denso maybe because its newer ,maybe not. Once warm and driving the 2 O2 sensors in my opinion perform identically. Why the change in code from p0420 to p0136?, idk but I have ordered a new Denso rear sensor to replace the possibly faulty one and will keep posted of my findings


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post #753 of 774 (permalink) Old 10-18-2016, 08:25 AM
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P0136 = Rear oxygen sensor circuit malfunction.

That is telling you that the rear O2 sensor is malfunctioning. That would explain why none of the "workarounds" you did, for the rear O2 sensor, failed to resolve your P0420 CEL code.

The sensor has to be 100% functional, before fixes like the $5 fix will work.

The curious thing is why the electrical fix you did didn't work?

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)
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post #754 of 774 (permalink) Old 11-02-2016, 07:32 PM
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We had been dealing with intermittent P0420 issues on my Tribeca pretty much all summer. I replaced the rear 02 sensor for about $50 bucks mid summer and no change was introduced and the issue persisted. At the end of September I replaced the rear O2 sensor again with a genuine OEM sensor for about $120. So far the problem has not returned.
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post #755 of 774 (permalink) Old 11-05-2016, 08:23 PM
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That's good news, after all my detailed approaches I felt unsatisfied. I picked up a new up stream and down stream sensor from Denso, 584 miles with no mil, which leaves me feeling hopeful. Thank you for the help!

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post #756 of 774 (permalink) Old 11-16-2016, 04:28 PM
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Got a PO420 today on my '15 FXT.

Reset it myself at the moment.
Just filled up with gas, engine oil temp was 124*, lead food take off at light.
42k miles. Worried some of the chems I used to clean the carbon might be causing an issue.

Not having great luck with this car.

3 flat tires in two months (warranty covered two)
Ran off the road and $3k in damage, no witnesses so $500 deductable.
Towed twice, once for accident. Once for dealer not properly tightening air hoses. That one hurt cause we had to take a 2week trip in a friend's parents car...
Car has been hit in parking lots now (different ones, vastly different places) 7 times. Dents on every panel.
I've had 4 cars. Been driving for 20 years. Never had this many issue with a car, I'm thinking the law of averages is catching up with me.

Thinking about trading for a 3.6r outback...
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post #757 of 774 (permalink) Old 12-24-2016, 03:28 PM
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P0420 questions

I've been getting a 420 code for a while. No exhaust leaks that I could find initially, swapped rear o2, and neither fixed the issue. So that basically leaves me with having to buy a new cat. How do I know which one I should replace? There are 2 according to several diagrams my downpipe and midpipe. Also recently I've developed a leak and it will occasionally bog down. I'm pretty sure that is related to this issue. At the cost of each cat I'm hesitant to just start part swapping and hope for the best.
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post #758 of 774 (permalink) Old 12-24-2016, 04:51 PM
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Quote:
Originally Posted by shag_fu View Post
How do I know which one I should replace? There are 2 according to several diagrams my downpipe and midpipe.
This should help with the diagnosis:


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post #759 of 774 (permalink) Old 12-24-2016, 04:52 PM
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@shag_fu I have some questions:

• Is your Forester an X or XT?
• You wrote that you don't have any exhaust leaks, but ended saying you recently developed a leak?
• Did you replace the rear O2 sensor with a factory/OEM O2 sensor?
• Any other CEL codes, other then the P0420?
• How's the engine performance & gas mileage?

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 #760 of 774 (permalink) Old 12-24-2016, 07:21 PM
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Forester is an XT. I originally posted in that thread so I didn't think I needed to state that explicitly when I started that thread.
When the code started I didn't have any exhaust leaks. I started to try or loosen bolts and broke one just recently and the leak showed up at the joint I broke a bolt. So I don't think that started my issue.
No not an oem sensor.
420 is my only code.
Mileage is 15-20 something depending how much highway driving I do. It's my GF car so I'm not tracking the mileage like I do on my truck. It had a turbo code for the little screen in the banjo bolt being clogged and a gas cap code a few months ago,but those haven't showed up since September.

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post #761 of 774 (permalink) Old 12-24-2016, 08:04 PM
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All threads/posts are subject to review & movement for the best fit.

It's recommended that the "public Profile" be filled out as that information appears under your user name, every time you post. That information could mean getting member replies, or not...
@pleiad7 posted a link above to diagnose a bad CAT. Have you viewed the video & do you think it's a bad CAT?

You might also peruse this thread: $5 fix

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 #762 of 774 (permalink) Old 12-25-2016, 09:12 AM
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I did watch that video but not recently. I did monitor the voltage of the 2 sensors. I didn't check the temp diff as I don't have a temp gauge like that. That was a month or 2 ago so I really need to revisit that process. Would the temp thing give me a more definitive answer?
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post #763 of 774 (permalink) Old 01-01-2017, 09:44 AM
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My 2015 FXT gave me CEL, check emissions control, check hill assist, eye sight disabled, traction control disabled light show two nights ago. Picked up error code reader at Pep Boys and have the P0420 Code displayed when I read the data. Car has ~22k miles on it, I treat her right as far as high octane gas, subaru service, etc. Have a service appt set up for Tues morning, going to see how this all plays out. Anything I can add here to help everyone out w/similar problems in the future let me know, if there'd add'l info to collect. Will update the thread after service.
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post #764 of 774 (permalink) Old 01-02-2017, 01:12 PM
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^^ you need a new front cat. under warrenty.

TrailAngel
2010 2.5X 5MT
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post #765 of 774 (permalink) Old 01-17-2017, 11:11 AM
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2010 Forester X 2.5, I got the dreaded CEL and showed a P0420 code on my OBD scanner 24 hours after having my spark plugs, wires and Coil Pack replaced. That was a month ago when it broke down on the highway and had to get towed to a dealer, my Christmas Holiday got a whole lot more exciting than I wanted it to. I am finally taking it to the dealer for P0420. Service guy on the phone thinks the flame went right through and into the Catalytic Converter during the misfiring of the bad spark plugs, he also said I would get a new Catalytic Converter under the warranty 8 years or 130,000 km. We shall see what happens on Saturday when the Service guy at the Dealer looks at it.
98,000 km and 6 years old.

Last edited by Break68; 01-17-2017 at 11:26 AM.
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