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2006 Subaru Outback AWD Manual, 2.5i Limited SOHC
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28 Posts
Discussion Starter · #1 ·
Hi friends,

I recently had my first run-in with the infamous P0420 code (Catalyst System Efficiency Below Threshold) on my 2006 Outback 2.5 Limited. The CEL has been on with the P0420 code for probably about a month now. At first, it was intermittent and would sometimes go off on its own (I didn’t clear the code or anything).

I initially thought that maybe this could indicate poor fuel quality, especially since it started about the time that the switch to winter-blend fuel would be happening. However, the code is now steadily showing and has been on for a few weeks without going off. In addition, I always use Costco Gasoline, which I believe is fairly good quality.

Now I’m on a mission to figure out what the problem is and would be grateful for any and all help! Today I finally got around to taking a nice drive to collect some data. I use a WIFI OBDII scanner and free app which surprisingly lets me collect and record data from a bunch of sensors and export it all as a CSV spreadsheet. The data shows a little over 10 minutes of driving at varying speeds from 0-50 mph. The engine was up to temp for the whole drive (but this is all shown in the data).

I imported everything into excel and linked the file here (apparently I can't attach excel files to a post). The file shows all recorded sensor data as well as “sensors of interest” interest being further analyzed in the following format:

  • The first sheet is called “Full Data.” This shows everything in raw data format, and the “PID” column can be filtered to look at any specific sensor.
  • The following sheets are the ones that I processed further based on which I thought were most relevant. For these sensors, I isolated the data and plotted it with labels so everything is clear. These sheets are as follows, in this order:
    • Vehicle Speed
    • Engine RPM
    • Engine Coolant Temp
    • Oxygen Sensor 1 Wide Range Voltage
    • Oxygen Sensor 2 Bank 1 Voltage
    • Oxygen Sensor 1 Wide Range Current
    • Oxygen Sensor 2 Wide Range Equivalence Ratio
    • Oxygen Sensor 2 Bank 1 Fuel Trim
    • Short Term Fuel % Trim – Bank 1
    • Long Term Fuel % Trim – Bank 1
    • MAF Rate
    • Vehicle Acceleration
I think this is probably overkill in some ways, but I secretly have a sick and twisted side that likes preparing data. I’m sort of young and dumb, so I’m hoping someone who has experience with this code can weigh in and point me in the right direction.

I’m trying to avoid blindly throwing a bunch of money at things to see if it fixes the problem. I’d like to systematically isolate the most likely causes and get this code out of here. The last thing I’ll say is that I really don’t want to use a cheater extender or something to trick the code. I know that will get me through inspection (yes, emissions checks suck here), but I don’t like “fixing” things that way.

Here is the link to the file again: https://drive.google.com/open?id=1YOe7I1p2MXnPR3y_dtaeMlBuVgLdM1me
If it's too cumbersome to download or it's not working, please let me know and I can post some screenshots instead.

Thanks!
 

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2005 Outback VDC limited 3.0r
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1,132 Posts
It looks like your rear 02 sensor is not reading at all during some points. General rule for 02 sensors is .1v(100mv)-.9v(900mv). Then the front is a wideband sensor and appears to be reading correctly.
 

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2006 Subaru Outback AWD Manual, 2.5i Limited SOHC
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28 Posts
Discussion Starter · #3 ·
It looks like your rear 02 sensor is not reading at all during some points.
Thanks, I will hopefully have some time soon to pull the sensor out and see if I can find a definite issue.
 

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18,866 Posts
It looks like your rear 02 sensor is not reading at all during some points. General rule for 02 sensors is .1v(100mv)-.9v(900mv). Then the front is a wideband sensor and appears to be reading correctly.
The Rear O2 sensor voltage will go to zero when coasting at speed, off throttle, in gear. In that situation, the drive-by-wire system will go into fuel cut (and "open loop"), so the engine is basically pumping air. That leads to the rear O2 sensor voltage being zero. This can be confirmed as during fuel cuts, the Air/Fuel ratio is very high. In other words, the exhaust is mainly air. This we see in the Sensor 1 Wide Range Eq graph.

However, the Sensor 2 Bank 1 (Rear O2 Sensor) voltage is varying a lot, and that can be indicative of a bad sensor or a bad cat.

p.s., https://www.subaruoutback.org/threads/p0420-diagnosis.49537 would be a good source of information on what to log, and to an extent, how to interpret the data. It's a long thread, but, I think, worthwhile reading. The P0420 can be challenging.
 

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2006 Subaru Outback AWD Manual, 2.5i Limited SOHC
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28 Posts
Discussion Starter · #5 ·
However, the Sensor 2 Bank 1 (Rear O2 Sensor) voltage is varying a lot, and that can be indicative of a bad sensor or a bad cat.
Is replacing the rear O2 sensor a reasonable place to start? Cardoc seems to indicate that the CAT is only the culprit in a handful of cases...
 

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2005 Outback VDC limited 3.0r
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1,132 Posts
I would start there. In rare instances I have seen cat failures. I also dont believe in replacing sensors as a "pair", that's just a way to cover a shops tech in case of a incorrect diagnosis. I would recommend a denso sensor. You can watch live data on a sensor to see if its reading steady or lagging. Lagging or inconsistent readings usually indicate a faulty sensor.
 
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