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Old 01-27-2008, 08:49 AM   #1
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Troubleshooting a P0420 Code

Have a 2001 Subaru Legacy Wagon with ~165K on it. History - Intermittently over the course of a few years, and always in the cold winter, I've had the CEL come on always with a P0420 code - Catalyst Efficiency Below Threshold (Bank 1). Reset and the code would disappear for at least a year. When it came on more frequent I spoke with the dealer and they said replace the Cat(s). Present - Mieneke did the Cat in early December for $200 less than the dealer wanted for just the part. Went a month and now have P0420 codes. Cleared and they return in a matter of days. Have had these read at both Auto Zone and Advanced Auto - both have recommended replacing the O2 sensor as that is the problem and not the Cat in their opinion. Cat is under warranty so I can always go back and have that done again. Anybody have similar problems and which O2 sensor might be the problem - I'm concluding the front?
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Old 02-02-2008, 10:00 PM   #2
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the 420 ONLY has to do with the rear O2 sensor after the cat.

you can replace the sensor since the cat is new OR do a search for rear o2 sensor fix. You'll end up using 2 sparkplug anti-foulers to move the sensor away from the exhaust stream. ~$15 vs. ~$150 for a new sensor.

We get that code when we remove cats, its a universal code for ALL OBDII compliant cars. The fix is also universal.
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Old 02-19-2008, 11:03 PM   #3
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It'll probably be the rear O2 sensor as that is used to measure cat efficiency. But if you haven't changed your front O2 sensor till now then that might be a good thing to change as well. You can hook the front one up to an oscilloscope and have a look at its response time, I just replaced mine and it's crossing the 0.45v line about 5 times more frequently. From memory the new O2 sensors have an average life of 130k km, or 80k miles.

As for OBDII and cat efficiency:
"The OBD II system compares O2 sensor readings upstream and downstream of the converter. If the downstream readings start to match those of the upstream O2 sensor, it indicates a drop off in operating efficiency and sets a catalyst code P0420."

Last edited by loccomoffo; 02-19-2008 at 11:10 PM. Reason: 130k km not miles :)
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Old 03-03-2008, 12:30 PM   #4
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you could have a crack in the exhaust pipe or they reused the gaskets when they replaced that cat. Exhaust leak could be the cause.
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Old 06-13-2008, 09:47 AM   #5
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Just to update the original thread. Ended up replacing both the front and back O2 sensors and the CEL has stayed off. Mistakenly ordered the back sensor when I thought I was ordering the front so I kept the one and properly ordered the other. On 2 occasions I read an additional code indicating knock sensor problems - bought that also but never installed. Appears the O2 issues were causing the knock sensor code. All is well with 174K and still going strong. Be a few years before I buy the next one.
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