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#4812091 - 07/12/18 09:48 AM Subaru Baja P0420 code question?
BAJA_05 Online   content


Registered: 02/04/18
Posts: 381
Loc: Berks County/Pa.
This morning the check engine light came on & stayed on my Subaru Baja in signature. Took it to Advance Autoparts and when they scanned it -- the P0420 code came up. I looked the code up on the computer and I immediately thought "oxygen sensor." Googled the code for the Baja that presently has 91,973 original miles and it stated -- "common misdiagnoses."

1)Ignition System
2)Fuel System
3)Oxygen Sensor
4)EGR Valve

Question being then -- what is the code referencing to then???


Edited by Kestas (07/12/18 11:42 AM)
Edit Reason: changed to proper code
_________________________
2017 VW Jetta SE TSI 1.4L Turbo (hers)
M1 Euro 0W40 & FU (19K)
2008 Acura TL 3.2L QSHM 5W20 & FU (103K)
2005 Subaru Baja AWD 2.5L N/A 5spd.
Rotella T6 5W30 & Wix (91K)

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#4812095 - 07/12/18 09:55 AM Re: Subaru Baja P0402 code question?? [Re: BAJA_05]
Bottom_Feeder Offline


Registered: 12/07/08
Posts: 3033
Loc: Occupied Virginia
P0402 or P0420?

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#4812097 - 07/12/18 10:04 AM Re: Subaru Baja P0420 code question?? [Re: BAJA_05]
BAJA_05 Online   content


Registered: 02/04/18
Posts: 381
Loc: Berks County/Pa.
Sorry Everyone -- I posted the wrong code. The code was P0420 and now it wont let me delete my thread.
_________________________
2017 VW Jetta SE TSI 1.4L Turbo (hers)
M1 Euro 0W40 & FU (19K)
2008 Acura TL 3.2L QSHM 5W20 & FU (103K)
2005 Subaru Baja AWD 2.5L N/A 5spd.
Rotella T6 5W30 & Wix (91K)

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#4812109 - 07/12/18 10:22 AM Re: Subaru Baja P0420 code question?? [Re: BAJA_05]
Bottom_Feeder Offline


Registered: 12/07/08
Posts: 3033
Loc: Occupied Virginia
Subarus from a while back had catalytic converter problems and Subaru replaced lots of them under warranty. I had to finally replace mine (on my dime) on my '01 Impreza RS after years of getting by with a code reset tool. Totally worth doing if you're keeping the car.

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#4812113 - 07/12/18 10:27 AM Re: Subaru Baja P0420 code question?? [Re: Bottom_Feeder]
BAJA_05 Online   content


Registered: 02/04/18
Posts: 381
Loc: Berks County/Pa.
Do you know what mileage they were covering up to possibly and thank you for the feedback.
_________________________
2017 VW Jetta SE TSI 1.4L Turbo (hers)
M1 Euro 0W40 & FU (19K)
2008 Acura TL 3.2L QSHM 5W20 & FU (103K)
2005 Subaru Baja AWD 2.5L N/A 5spd.
Rotella T6 5W30 & Wix (91K)

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#4812174 - 07/12/18 11:12 AM Re: Subaru Baja P0420 code question?? [Re: BAJA_05]
Charlie2015 Offline


Registered: 04/24/15
Posts: 258
Loc: Georgia
If P0420 is the only code stored, you will probably need to replace the cat. Converters from all auto manufacturers are covered under federal warranty for 8 years or 80,000 miles - you are over on both counts. You can always call your local Subaru dealer and see if they can do any goodwill for you.
_________________________
1998 Honda Accord EX 2.3L Auto
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#4812190 - 07/12/18 11:28 AM Re: Subaru Baja P0420 code question?? [Re: Bottom_Feeder]
maxdustington Offline


Registered: 01/21/17
Posts: 843
Loc: Toronna
When I got my first car, a 2000 Golf 2.0 it would throw that code every so often. I just kept clearing it. It came up a few times when I first got it, and the maybe twice over the next two years. I think if the cat efficiency is even 1% out of spec, it will throw the code. I've seen a few cars throw it seemingly randomly.

Originally Posted By: Bottom_Feeder
Subarus from a while back had catalytic converter problems and Subaru replaced lots of them under warranty. I had to finally replace mine (on my dime) on my '01 Impreza RS after years of getting by with a code reset tool. Totally worth doing if you're keeping the car.

I would get a code reset tool (I have a Scangauge II) and see if it keeps coming up. If it comes up often, you can start other diagnostics such as monitoring the front and rear O2 sensors to see if it is the cat or the rear O2 sensor.
_________________________
03 Jetta AWP/09A 200k kms
Edge 0W40 + Mann 719/30

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#4812218 - 07/12/18 12:10 PM Re: Subaru Baja P0420 code question?? [Re: BAJA_05]
JMJNet Offline


Registered: 09/06/07
Posts: 950
Loc: TX, USA
In Toyota, most of the time, the after cat O2 sensor is bad.
I have had 3 occurences with my own and friends' Toyota with P0420.
Not sure if the Subaru is like that though.

Charlie (above) suggested 8 years 80k warranty, from signature it is a 2005 so well pass 8 years warranty.
Unless they have an extended recall warranty, then you are not covered anymore.
_________________________
  • 2017 Toyota Sienna
  • 1995 GMC Suburban 6.5L Turbo Diesel



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#4812252 - 07/12/18 12:54 PM Re: Subaru Baja P0420 code question?? [Re: BAJA_05]
JTK Offline


Registered: 08/14/03
Posts: 10653
Loc: Buffalo, NY
Like any scenario where a catalyst inefficiency code keeps happening, once you rule out the exhaust system is solid and there's no vacuum leaks, you can try replacing O2 sensors. New ones might show the proper delta. If that doesn't work, you can try stacking some spark plug antifoulers in the downstream bung. All that does is push the sensor away from the exhaust stream a bit so it hopefully detects less O2 than the front sensor as it should. The book fix is to replace the catalytic converter(s).

FWIW, I've seen lots of family and friends older, high mileage Subarus throw this code.


Edited by JTK (07/12/18 12:56 PM)
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2017 Ram 1500 4x4, 3.6L. 2016 Nissan Quest SV (Babe magnet IV)

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#4812281 - 07/12/18 01:30 PM Re: Subaru Baja P0420 code question?? [Re: BAJA_05]
Dylan1303 Offline


Registered: 10/14/15
Posts: 200
Loc: Washington
My old tiburon had no cats. I would always trigger a 420 code when i used safeway gas. Did u try a new gas station?
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12 cruze 1.4t 6spd pp 5w30 tuned on 22lbs
13 equinox 2.4 awd pp 5w30

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#4812360 - 07/12/18 03:11 PM Re: Subaru Baja P0420 code question?? [Re: BAJA_05]
sxg6 Offline


Registered: 05/23/04
Posts: 2481
Loc: Northeast
Catalytic converter or bad rear oxygen sensor falsely saying the cat isn't operating efficiently.

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#4812387 - 07/12/18 03:39 PM Re: Subaru Baja P0420 code question?? [Re: JTK]
dparm Offline


Registered: 04/19/10
Posts: 13512
Loc: Chicago, IL
Originally Posted By: JTK
Like any scenario where a catalyst inefficiency code keeps happening, once you rule out the exhaust system is solid and there's no vacuum leaks, you can try replacing O2 sensors. New ones might show the proper delta. If that doesn't work, you can try stacking some spark plug antifoulers in the downstream bung. All that does is push the sensor away from the exhaust stream a bit so it hopefully detects less O2 than the front sensor as it should. The book fix is to replace the catalytic converter(s).

FWIW, I've seen lots of family and friends older, high mileage Subarus throw this code.



I would agree with this.

If the sensor isn't seized up, try removing it and cleaning with electronics cleaner. Inspect the sensor, harness, and wires for obvious signs of damage. Sometimes, depending on the car, the front and rear sensors might be identical -- in that case, swap them and see if the code changes.
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2017 Chevrolet Corvette Grand Sport

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#4812390 - 07/12/18 03:41 PM Re: Subaru Baja P0420 code question?? [Re: BAJA_05]
Loi Offline


Registered: 05/14/11
Posts: 46
Loc: Des Moines, IA
P0420 is usually a catalytic converter or a rear o2 sensor not completing the heated circuit. Very common on high mileage Subarus or on cars with modified exhaust systems with no catalytic converters.
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'05 GDB STI
'16 VAB STI

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#4812420 - 07/12/18 04:05 PM Re: Subaru Baja P0420 code question?? [Re: BAJA_05]
Loi Offline


Registered: 05/14/11
Posts: 46
Loc: Des Moines, IA
For a Subaru, you cannot swap the front o2 sensor with the rear o2 sensor. The front o2 sensor on Subarus is the factory air fuel ratio sensor mounted on the exhaust manifold.
_________________________
'05 GDB STI
'16 VAB STI

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#4812677 - 07/12/18 08:41 PM Re: Subaru Baja P0420 code question?? [Re: BAJA_05]
BAJA_05 Online   content


Registered: 02/04/18
Posts: 381
Loc: Berks County/Pa.
What name brand are the best sensors to buy/replace the factory oxygen sensors? Plan on replacing upstream & downstream all at one shot.
_________________________
2017 VW Jetta SE TSI 1.4L Turbo (hers)
M1 Euro 0W40 & FU (19K)
2008 Acura TL 3.2L QSHM 5W20 & FU (103K)
2005 Subaru Baja AWD 2.5L N/A 5spd.
Rotella T6 5W30 & Wix (91K)

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