Subaru Baja P0420 code question?

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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???
 
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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.
 
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.
 
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.
 
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.
 
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.
 
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Catalytic converter or bad rear oxygen sensor falsely saying the cat isn't operating efficiently.
 
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.
 
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.
 
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.
 
What name brand are the best sensors to buy/replace the factory oxygen sensors? Plan on replacing upstream & downstream all at one shot.
 
Originally Posted By: AVB
I would go with NTK or Denso.


Agreed, huge OEM suppliers. Bosch should be okay too.
 
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