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Subaru Baja P0420 code question? #4812091
07/12/18 08:48 AM
07/12/18 08:48 AM
Joined: Feb 2018
Posts: 509
Chester County/Pa.
BAJA_05 Offline OP
BAJA_05  Offline OP
Joined: Feb 2018
Posts: 509
Chester 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???

Last edited by Kestas; 07/12/18 10:42 AM. Reason: changed to proper code

2017 VW Jetta SE TSI 1.4L Turbo (hers)
M1 Euro 0W40 & FU (19K)
2008 Acura TL 3.2L PPPP 0W20 & FU (108K)
2005 Subaru Baja AWD 2.5L N/A 5spd.
QSHM 5W30 & FU (96K)
2000 Ram 4X4 5.9L (93K) PYB 10W30 & FU
Re: Subaru Baja P0402 code question?? [Re: BAJA_05] #4812095
07/12/18 08:55 AM
07/12/18 08:55 AM
Joined: Dec 2008
Posts: 3,114
Occupied Virginia
Bottom_Feeder Offline
Bottom_Feeder  Offline
Joined: Dec 2008
Posts: 3,114
Occupied Virginia
P0402 or P0420?

Re: Subaru Baja P0420 code question?? [Re: BAJA_05] #4812097
07/12/18 09:04 AM
07/12/18 09:04 AM
Joined: Feb 2018
Posts: 509
Chester County/Pa.
BAJA_05 Offline OP
BAJA_05  Offline OP
Joined: Feb 2018
Posts: 509
Chester 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 PPPP 0W20 & FU (108K)
2005 Subaru Baja AWD 2.5L N/A 5spd.
QSHM 5W30 & FU (96K)
2000 Ram 4X4 5.9L (93K) PYB 10W30 & FU
Re: Subaru Baja P0420 code question?? [Re: BAJA_05] #4812109
07/12/18 09:22 AM
07/12/18 09:22 AM
Joined: Dec 2008
Posts: 3,114
Occupied Virginia
Bottom_Feeder Offline
Bottom_Feeder  Offline
Joined: Dec 2008
Posts: 3,114
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.

Re: Subaru Baja P0420 code question?? [Re: Bottom_Feeder] #4812113
07/12/18 09:27 AM
07/12/18 09:27 AM
Joined: Feb 2018
Posts: 509
Chester County/Pa.
BAJA_05 Offline OP
BAJA_05  Offline OP
Joined: Feb 2018
Posts: 509
Chester 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 PPPP 0W20 & FU (108K)
2005 Subaru Baja AWD 2.5L N/A 5spd.
QSHM 5W30 & FU (96K)
2000 Ram 4X4 5.9L (93K) PYB 10W30 & FU
Re: Subaru Baja P0420 code question?? [Re: BAJA_05] #4812174
07/12/18 10:12 AM
07/12/18 10:12 AM
Joined: Apr 2015
Posts: 287
Georgia
Charlie2015 Offline
Charlie2015  Offline
Joined: Apr 2015
Posts: 287
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 (Sold) 214k mi
2009 Scion tC Manual (Daily Driver) 165k mi
Re: Subaru Baja P0420 code question?? [Re: Bottom_Feeder] #4812190
07/12/18 10:28 AM
07/12/18 10:28 AM
Joined: Jan 2017
Posts: 1,814
VW Fanboy Island
maxdustington Online content
maxdustington  Online Content
Joined: Jan 2017
Posts: 1,814
VW Fanboy Island
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 205k kms
Edge 0W40 + Mann 719/30
Re: Subaru Baja P0420 code question?? [Re: BAJA_05] #4812218
07/12/18 11:10 AM
07/12/18 11:10 AM
Joined: Sep 2007
Posts: 1,012
TX, USA
JMJNet Offline
JMJNet  Offline
Joined: Sep 2007
Posts: 1,012
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


Re: Subaru Baja P0420 code question?? [Re: BAJA_05] #4812252
07/12/18 11:54 AM
07/12/18 11:54 AM
Joined: Aug 2003
Posts: 10,946
Buffalo, NY
JTK Offline
JTK  Offline
Joined: Aug 2003
Posts: 10,946
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.

Last edited by JTK; 07/12/18 11:56 AM.

2017 Ram 1500 4x4, 3.6L. 2016 Nissan Quest SV (Babe magnet IV)
Re: Subaru Baja P0420 code question?? [Re: BAJA_05] #4812281
07/12/18 12:30 PM
07/12/18 12:30 PM
Joined: Oct 2015
Posts: 280
Washington
Dylan1303 Offline
Dylan1303  Offline
Joined: Oct 2015
Posts: 280
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?


12 cruze rs 1.4t 6spd pp 5w30 tuned on 22psi
13 equinox 2.4 awd pp 5w30
Re: Subaru Baja P0420 code question?? [Re: BAJA_05] #4812360
07/12/18 02:11 PM
07/12/18 02:11 PM
Joined: May 2004
Posts: 2,493
Northeast
sxg6 Offline
sxg6  Offline
Joined: May 2004
Posts: 2,493
Northeast
Catalytic converter or bad rear oxygen sensor falsely saying the cat isn't operating efficiently.

Re: Subaru Baja P0420 code question?? [Re: JTK] #4812387
07/12/18 02:39 PM
07/12/18 02:39 PM
Joined: Apr 2010
Posts: 13,537
Chicago, IL
dparm Offline
dparm  Offline
Joined: Apr 2010
Posts: 13,537
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.


2017 Chevrolet Corvette Grand Sport
Re: Subaru Baja P0420 code question?? [Re: BAJA_05] #4812390
07/12/18 02:41 PM
07/12/18 02:41 PM
Joined: May 2011
Posts: 55
Des Moines, IA
Loi Offline
Loi  Offline
Joined: May 2011
Posts: 55
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.


'05 GDB STI
'16 VAB STI
Re: Subaru Baja P0420 code question?? [Re: BAJA_05] #4812420
07/12/18 03:05 PM
07/12/18 03:05 PM
Joined: May 2011
Posts: 55
Des Moines, IA
Loi Offline
Loi  Offline
Joined: May 2011
Posts: 55
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
Re: Subaru Baja P0420 code question?? [Re: BAJA_05] #4812677
07/12/18 07:41 PM
07/12/18 07:41 PM
Joined: Feb 2018
Posts: 509
Chester County/Pa.
BAJA_05 Offline OP
BAJA_05  Offline OP
Joined: Feb 2018
Posts: 509
Chester 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 PPPP 0W20 & FU (108K)
2005 Subaru Baja AWD 2.5L N/A 5spd.
QSHM 5W30 & FU (96K)
2000 Ram 4X4 5.9L (93K) PYB 10W30 & FU
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