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Old 12-13-2012, 11:43 PM   #1
Sir-Knight
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Default p0420 front o2 dead?

refresh my memory, p0420 is a front o2 sensor code right? I think I might need to replace it again.
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Old 12-14-2012, 12:03 AM   #2
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No that is a code for your cat not working right. Rear 02 sensor.
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Old 12-14-2012, 07:41 AM   #3
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That is a catalytic converter efficiency code. You are going to need a converter.
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Old 12-14-2012, 09:34 AM   #4
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Quote:
Originally Posted by SubaruTech84 View Post
That is a catalytic converter efficiency code. You are going to need a converter.

P0420 is more than just a cat issue. It is saying the readings between the upstream O2 sensor is different than the readings from the downstream O2 sensor.

It CAN be cat, but it could be lower O2 failure, a header leak, unburnt particulates in the exhaust, etc.


How many miles are on the car?


FWIW, I have 155k on my 2.5L and have a P0420, after having the front O2 sensor fault fixed. Replaced rear sensor and cat all the gaskets between header and cat-back. ~$900 in parts and it still hasn't cleared.

Last edited by ADaughen; 12-14-2012 at 09:45 AM.
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Old 12-14-2012, 06:23 PM   #5
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100k, km the cat is has about 40-50k on it... aftermarket EL header 2.5" all the way back... threw the code with the 2.5" tin can muffler, and also with a 2.25" wrx muffler/axleback.

The car ate a front o2 sensor already with the stock exhaust.. the original front o2 lasted about 5k on the wider pipe before it died. That's why I suspect the front o2.
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Old 12-15-2012, 10:55 AM   #6
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P0420 diag. is pretty straight forward. If the rear O2 sensor is responding and changing values when you force a rich and lean condition by adding combustibles and creating a vacuum leak odds are 100% the cat is bad.

Warranty procedure calls for you to record front and rear sensors under specific drive conditions and compare the readings. What you are looking for is the ability of the cat to store O2. The recording is saved in case Subaru kicks the claim. You can see the change while driving but you should have your eyes on the road hence the recording.
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Old 12-15-2012, 01:47 PM   #7
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Yeah, if the front O2 (air/fuel sensor) was bad, your car would run horrid. It takes a LOT to trigger a fault in the front O2, enough so where the car is almost undrivable. You would have big hesitation issues, like it was misfiring, crap idle that would idle high, drop down and almost die, probably die at times at stop lights if you weren't revving the engine a good deal to keep inertia. You'd have a very hard time holding a constant speed on the highway to within +/- 5mph because you'd have vary the throttle so much all the time. The only way the car is driveable is to basically floor it, coast, floor it, coast, and repeat till you get to your destination. You have to force the car to be in open loop to actually use the car, and this means heavy throttle. It's the only time the car ignores the sensor and just goes off ECU mapping. It will also ignore the sensor on initial startup during the warm up phase. A sign of a bad front O2 (air/fuel sensor) is fine cold start, fine heavy throttle, and crap everything else, and it just keeps getting worse until the car is almost undriveable outside of forcing open loop through heavy throttle use.
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Old 12-17-2012, 03:08 PM   #8
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when the front o2 went bad the first time, idle was hunting a lot and on throttle off rpms would just die and the engine would cut. It happened a couple times coming up to a stop. But was a different code. I completely unplugged the front 02 and replaced it with a known dead one just to keep the hole plugged while waiting for a replacement. The ecu was able to compensate and use the rear o2 and the car drove normally after it got used to it. Of course, the low voltage code from the front o2 was constantly on until the sensor was replaced. After that it ran fine unless I dumped a lot of fuel when the engine/exhaust was cold (-20C)


This time around, with 420, I would clear it and it would exhibit the same throttle off rpm dive until it relearned the mapping. Light hasn't come back on in a week. Temps this time around (threw 420 twice in the last couple months) were just above and below freezing.)

if 420 is primarily a rear o2 sensor or cat code, is it more than likely the rear o2 needs replacement, seeing the cat is fairly new?
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Old 12-17-2012, 03:35 PM   #9
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Mr bass gives a accurate explanation of how it works on post #6. The rear 02 is switching with the ecu from rich to lean and the cat isn't cleaning it up, so it throws a code. If it was bad it wouldn't be switching and you wouldn't get a p0420 code.
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Old 12-18-2012, 03:08 PM   #10
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so more than likely it doesn't think the cat is doing a good job? It isn't an oem cat by any means, it's an oem replacement with a 2.5" in/out... but judging by the number of 420 threads posted where people have replaced cat and sensors and are still dealing with the code, I'm hesitant to throw money at replacing the cat.
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Old 12-18-2012, 06:39 PM   #11
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Quote:
Originally Posted by Sir-Knight View Post
so more than likely it doesn't think the cat is doing a good job? It isn't an oem cat by any means, it's an oem replacement with a 2.5" in/out... but judging by the number of 420 threads posted where people have replaced cat and sensors and are still dealing with the code, I'm hesitant to throw money at replacing the cat.

Yes that is what the code indicates. The thing about it is you have to also be sure you don't have a misfire, a bad plug, too much ethanol, a burnt valve or some other issue causing the engine to run rich and waste the cat. A bad A/F (front "O2" sensor) can waste a cat. You have to determine what caused the cat and/or sensors to fail to ensure it doesn't happen again. You have to look at the whole picture, not just the code.

Odds are if you replace a part and it fails again then you didn't do a comprehensive or proper diagnoses and you did not address the root cause.
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Old 12-18-2012, 09:53 PM   #12
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If the cat was an inexpensive one they have less precious metals in them so they don't last as long.
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Old 12-19-2012, 08:55 AM   #13
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seeing the code has only been thrown a very few times, and not constantly, I don't think I'll worry about it right now then... if it starts coming up a lot more than I'll have no choice, but I'm reasonably sure my engine is healthy, and that it's just an occasional 420 that aftermarket exhaust users get.
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Old 12-19-2012, 10:57 AM   #14
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That code sucks. I switched my exhaust and had the same issues. Lucky I have Torque Pro and a Bluetooth obd 2 so I just clear it when. It pops up.
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Old 01-11-2013, 08:29 AM   #15
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Default P0420 code...

New to the forum. I was given a 97 Outback Sport and getting the dreaded P0420 code trying to pass emissions. I read on a previous post about using a spacer on the rear O2 Sensor and it might pass. Does anyone have experience with this spacer and where I could I get one?

I know I should diagnose the whole system, look for leaks etc. but I thought this may be a real cheap way to try and knock this out. Thanks.
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Old 01-11-2013, 08:52 AM   #16
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Autozone or Advance auto parts sell the spark plug deflouler

http://www.autozone.com/autozone/par...uler/_/N-9n8cq

Just make sure to ask that it fits your car. You only need one.




The funny thing is today I got my CEL, pretty sure it is my P0420
I need to change all my exhaust from header down to muffler. It is in very bad shape, full of leaks and the joints look like they will crumble once I take the bolts apart.
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Old 01-11-2013, 09:07 AM   #17
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Thanks for the info. Gonna pick one up today and see if I am able to get the sensor off after a good soaking of Liquid Wrench.....
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