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    Thread: Solution to the rear O2 CEL (P0420)

    1. 04-29-2004 09:27 AM #1
      Finally... it has arrived!
      Non-permanent mechanical solution to the rear O2 CEL (P0420)

      [SIZE=big]CEL free for 12,000 miles and counting! (as of 5/23/05)[/SIZE]
      First of all, let me thank our scooby friends for the idea:
      http://forums.nasioc.com/forum...l+fix
      Basically, this is a solution to the dreaded CATALYST EFFICIENCY BELOW THRESHOLD CEL (P0420, DTC 16804) that many of us get from upgrading the downpipe. The idea is to place a gap between the pipe and the sensor to reduce the amount of exhaust gases that the sensor will see/detect. This becoming a very popular mod over on the Impreza boards since it is NON-PERMANENT, and it does not require modification to the O2 sensor harness. Normally, I would throw a CEL about every 85 miles, so far this mod has been worthwhile.
      Parts/tools Required:
      1) Help! Part no. 42002 and/or 42009 by Motormite, these are known as sparkplug-arresters, which should be available at any generic auto supply store (e.g. Pepboys, Autozone, etc.). Should run about $5 per package.
      2) Tools
      - means to lift/secure vehicle
      - 10 mm socket or wrench
      - 7/8” wrench for O2 sensor (or 22 mm)
      - drill and 1/2” drill bit (read below)
      - Dremel tool and various bits
      - zip ties & dykes
      3) (most defly)
      Procedure:
      1) Jack up car and secure with jack stands.
      2) Remove 2 plastic nuts and shield covering the O2 sensor/harness junction (10mm), this will make the R&R of the rear O2 much easier.
      3) Remove the rear O2 sensor, which is the one downstream of the cat (7/8” or 22mm).
      4) Modify the sparkplug-arrester(s). A 1/2” drill bit did the job for the Scooby sensor, but the vdub sensors are setup differently.
      First I drilled out the arrester using the 1/2” drill bit, then I used various drill bits and the Dremel to shave the rest until the sensor would fit. If you have the means to measure the sensor and get the appropriate drill bit, that would be highly recommended.
      5) Install arrester(s) into downpipe & replace O2 sensor (7/8” or 22mm).
      From the pictures you can see that there are several combinations for the fix. The 42002 is a tapered-mount and the 42009 is a flat-mount with a crush washer. The 42002 (tapered) is notably longer in length than the 42009 (flat).
      I elected to start off with the 42002 (tapered) only, which is the longer of the two, this worked out fine for 120 miles. Next, I placed a non-drilled 42009 (flat) in front of the 42002, so far no problems.
      Another method for using two arresters in-line is only drill through both. Some Impreza owners have seen a SLOW RESPONDING O2 SENSOR CEL (P0139, DTC 16523) CEL. As you can see there may be some trial and error to find what’s right for your dub (specifically the cat or lack of in your downpipe).

      6) Replace 2 plastic nuts and shield covering the O2 sensor/harness junction (10mm).
      7) Secure harnesses with zip ties.
      8) Clear DTC’s/Reset ECU.
      That’s all folks! [IMG]http://**********************/smile/emthup.gif[/IMG]

      Help parts

      Help parts again

      42009 drilled

      42002 only

      42002 (drilled) & 42009


      [******]As always YOU assume all responsibility for this mod![/size]



      Modified by kdub790 at 3:52 PM 5-23-2005

    2. 04-29-2004 10:15 AM #2

      Looks good, hope it works for ya. Question if it affects any of the information the O2 sends to the ecu? Im gonna try this, I cant stand that cel anymore good write up [IMG]http://**********************/smile/emthup.gif[/IMG]


      Modified by jeller at 10:28 AM 4-29-2004

    3. 04-29-2004 11:19 AM #3
      Quote, originally posted by jeller »
      Question if it affects any of the information the O2 sends to the ecu?

      that is the point to doing this mod. basically the rear O2 sensor functions as a tattle-tail for whether the catalyst is working like the ecu thinks it should.
      perhaps this little write up will shed some light on the function of front and rear O2 sensors.
      click-me.

    4. Member Ken1.8T's Avatar
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      04-29-2004 11:33 AM #4
      This is an awesome mod, I can not wait to give it a try! Thank you!

    5. 04-29-2004 11:38 AM #5
      Quote, originally posted by kdub790 »
      that is the point to doing this mod. basically the rear O2 sensor functions as a tattle-tail for whether the catalyst is working like the ecu thinks it should.
      perhaps this little write up will shed some light on the function of front and rear O2 sensors.
      click-me.

      Thanks for the link on the O2 sensors, innformative, what I meant was will it affect it in any negative way? But now I understand it better [IMG]http://**********************/smile/emthup.gif[/IMG]

    6. Banned VDUBNDizzy's Avatar
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      04-29-2004 11:38 AM #6
      Interesting. My guess is the metal in the "spark plug arrestor " works as a heat sink around the o2 sensor, keeping it happy.

    7. 04-29-2004 11:47 AM #7
      Quote, originally posted by VDUBNDizzy »
      Interesting. My guess is the metal in the "spark plug arrestor " works as a heat sink around the o2 sensor, keeping it happy.

      exhaust gases still reach the sensor, just not as much as before.

    8. Banned VDUBNDizzy's Avatar
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      04-29-2004 11:49 AM #8
      But you're implying that less heat will keep the code from popping. But it's the opposite that should be true.

    9. Member Power5's Avatar
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      04-29-2004 11:50 AM #9
      do you know what the output change is? I use that for my A/F gauge at the moment, so if this kills its accuracy, I dont want to do it. Just wondering.

    10. 04-29-2004 11:52 AM #10
      A heat sink would reduce the temp of the sensor which would not help.
      I think all it's doing is moving the sensor further away from the exhaust stream. Thereby reuducing the concentration of pollutants being sensed by the sensor.
      So the sensor is perceiving a cleaner exhaust just because it is moved further away.
      The trick is to get the right level of exhaust on that sensor because too little heat will throw a code. That's why you have to play with the fitment distance with multiple pieces to get the correct result.
      If this works, it leads me to believe that the cause of this code is not so much the temp of the sensor but, actual catalytic converters that aren't doing their job scrubbing the exhauts.

    11. Banned VDUBNDizzy's Avatar
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      04-29-2004 11:58 AM #11
      How would a heat sink that is wrapped around the sensor keep heat away from the sensor? And the 2nd o2 sensor doesn't sense any particular gases from the exhaust gas stream, it's just there for temperature purposes.

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      04-29-2004 12:04 PM #12
      The reason the CEL pops is that the CAT doesn't always get hot enough (efficient DP and all) to catalyze all the bad stuff and the rear sensor sniffs that out.
      The rear o sensor is heated by wire so that it will work. What he has done is remote the sensor enough so that it is not sniffing the bad stuff going by. Hence no CEL.
      The front o sensor controls mixture with the ecu so mixture and performance is not affected.
      The only effect in remoting the rear one a bit is it is not quite as sensitive now to the EG going by post CAT.

    13. 04-29-2004 12:08 PM #13
      wow! this thread is taking off.
      Power5... i haven't done any real data logging yet, but at idle i was at 0.47 volts (which maps to the Stoichiometric region) using the 2 arrester setup; i failed to do anything with the preivous 2 configurations.
      none the less, you shouldn't be using the sensor downstream of the cat for your a/f gauge!
      Dizzy.. the code appears from an insufficient difference in the front and rear sensors.



      Modified by kdub790 at 8:05 AM 4-30-2004

    14. 04-29-2004 12:16 PM #14
      Quote, originally posted by kdub790 »
      I elected to start off with the 42002 (tapered) only, which is the longer of the two, this worked out fine for 120 miles.

      So you added the second one because you were getting a CEL? Or were you just experimenting?
      GREAT FIND!!!

    15. 04-29-2004 12:17 PM #15
      Quote, originally posted by slugII »
      A heat sink would reduce the temp of the sensor which would not help.
      I think all it's doing is moving the sensor further away from the exhaust stream. Thereby reuducing the concentration of pollutants being sensed by the sensor.
      So the sensor is perceiving a cleaner exhaust just because it is moved further away.
      The trick is to get the right level of exhaust on that sensor because too little heat will throw a code. That's why you have to play with the fitment distance with multiple pieces to get the correct result.
      If this works, it leads me to believe that the cause of this code is not so much the temp of the sensor but, actual catalytic converters that aren't doing their job scrubbing the exhauts.

      what he said. [IMG]http://**********************/smile/emthup.gif[/IMG] [IMG]http://**********************/smile/emthup.gif[/IMG]
      Dizzy...there is not heatsink here, just adding a gap. instead of the sensor sitting directly in the flow, it has been recessed.

    16. 04-29-2004 12:19 PM #16
      Quote, originally posted by SlvrBllt »
      So you added the second one because you were getting a CEL? Or were you just experimenting?
      GREAT FIND!!!

      i still had those 420 blues.
      the second one went in unmolested (no drilling). some of the wrx guys drill through both.

    17. 04-29-2004 12:46 PM #17

      so I should only drill out one of them or both of them? only one of yours is drilled, correct?


      Modified by jeller at 12:49 PM 4-29-2004

    18. 04-29-2004 12:51 PM #18
      Soo you tired out the 42009(drilled) and then your got a cell at 120 miles. Then you just added the 42002(the way it is) in front and Eureka ! ! ! Sweeeet. I'm sooo down to try this. Soo in total it was like 10 bucks? Let us know....how many miles now...

    19. 04-29-2004 12:52 PM #19
      the base is not drilled, the one the sensor goes in has to be drilled for obvious reasons.

    20. 04-29-2004 12:54 PM #20

      Thanks, great info, hope this works, It will make me soo happy not to have to look at that fooking light anymore

    21. 04-29-2004 12:55 PM #21
      Quote, originally posted by BoostedGolf02 »
      Soo you tired out the 42009(drilled) and then your got a cell at 120 miles. Then you just added the 42002(the way it is) in front and Eureka ! ! ! Sweeeet. I'm sooo down to try this. Soo in total it was like 10 bucks? Let us know....how many miles now...

      i believe one package of either the 42009 or 42002 was about $5.50 here in dallas. i went ahead and purchased one of each which gave me 4 total pieces. what i might reccomend is to use the 42009's (the one's with the crush gasket and are shorter in length), with one drilled and one not... this will maximize the ground clearance.


      Modified by kdub790 at 12:02 PM 4-29-2004

    22. 04-29-2004 02:09 PM #22
      Quote, originally posted by Power5 »
      do you know what the output change is? I use that for my A/F gauge at the moment, so if this kills its accuracy, I dont want to do it. Just wondering.

      wouldnt it make more sense to utilize the front o2 sensor as the data the rear o2 sensor reads has already been "tampered" with by the cat? i would assume that the "data" the a/f guage would display should be unaltered as it leaves the engine...

    23. Member JettaRed's Avatar
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      04-29-2004 02:47 PM #23
      Quote, originally posted by Power5 »
      do you know what the output change is? I use that for my A/F gauge at the moment, so if this kills its accuracy, I dont want to do it. Just wondering.

      You should be using your front o2 sensor for your a/f gauge; not the rear one.
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    24. Member dex's Avatar
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      04-29-2004 02:56 PM #24
      This deserves a place in the FAQ! [IMG]http://**********************/smile/emthup.gif[/IMG]

    25. 04-29-2004 03:18 PM #25
      nice idea for those whose downpipes aren't functioning up to spec.... now how about those people that are running no cat? You guys think this will work also, anybody w/ no cat wanna give it a try?

    26. 04-29-2004 03:37 PM #26
      Quote, originally posted by turbotuner20V »
      nice idea for those whose downpipes aren't functioning up to spec.... now how about those people that are running no cat? You guys think this will work also, anybody w/ no cat wanna give it a try?

      ghettlo showed interest, may be he'll post some results.

    27. 04-29-2004 03:42 PM #27
      Quote, originally posted by JettaRed »
      You should be using your front o2 sensor for your a/f gauge; not the rear one.

      exactly my point above...

    28. 04-29-2004 08:04 PM #28
      Well I have an ATP 3" downpipe with racepipe section coming to me as we speak. I will definitely give this a go when I install the downpipe, only problem is I dont have vag-com to clear the light when it comes on. Anyone know of a cheap tool to reset the light?

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      04-29-2004 08:15 PM #29
      This is awesome and I'll be trying it out on my day off next week!!!
      I also heard of some JDM guys putting a heat type wrap around the rear O2 sensor - the part that actually reads the heat/gases, so it'd end up being INSIDE of your dp/test pipe. This supposedly fixed some CEL's for them. My reasoning would be that it 1) Decreases the gases sniffed by the sensore and 2) Probably kept more heat on the sensor which would fulfill that need too. I was gonna try it, but haven't got around to it and not exactly sure how I'd secure the wrap to the sensor. Any thoughts?

    30. 04-29-2004 09:03 PM #30
      Quote, originally posted by kdub790 »
      i believe one package of either the 42009 or 42002 was about $5.50 here in dallas. i went ahead and purchased one of each which gave me 4 total pieces. what i might reccomend is to use the 42009's (the one's with the crush gasket and are shorter in length), with one drilled and one not... this will maximize the ground clearance.

      Modified by kdub790 at 12:02 PM 4-29-2004

      Where exactly did u purchase these parts?

    31. 04-29-2004 09:34 PM #31

      Look again at first post, under parts and tools required

    32. Member JettaRed's Avatar
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      04-29-2004 10:18 PM #32
      The o2 sensor doesn't sniff pollutants; it senses oxygen. Hence, oxygen sensor.
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    33. Member evilpat's Avatar
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      04-29-2004 10:42 PM #33
      here is a little on how the sensors work for those that are confused . . .
      An Oxygen sensor is a chemical generator. It is constantly making
      a comparison between the Oxygen inside the exhaust manifold and air
      outside the engine. If this comparison shows little or no
      Oxygen in the exhaust manifold, a voltage is generated. The
      output of the sensor is usually between 0 and 1.1 volts. All
      spark combustion engines need the proper air fuel ratio to
      operate correctly. For gasoline this is 14.7 parts of air to one
      part of fuel. When the engine has more fuel than needed, all
      available Oxygen is consumed in the cylinder and gasses leaving
      through the exhaust contain almost no Oxygen. This sends out a
      voltage greater than 0.45 volts. If the engine is running lean,
      all fuel is burned, and the extra Oxygen leaves the cylinder and
      flows into the exhaust. In this case, the sensor voltage goes
      lower than 0.45 volts. Usually the output range seen seen is
      0.2 to 0.7 volts.
      The sensor does not begin to generate it's full output until it
      reaches about 600 degrees F. Prior to this time the sensor is
      not conductive. It is as if the circuit between the sensor and
      computer is not complete. The mid point is about 0.45 volts.
      This is neither rich nor lean. A fully warm O2 sensor *will not
      spend any time at 0.45 volts*. In many cars, the computer sends
      out a bias voltage of 0.45 through the O2 sensor wire. If the
      sensor is not warm, or if the circuit is not complete, the computer
      picks up a steady 0.45 volts. Since the computer knows this is
      an "illegal" value, it judges the sensor to not be ready. It
      remains in open loop operation, and uses all sensors except the
      O2 to determine fuel delivery. Any time an engine is operated
      in open loop, it runs somewhat rich and makes more exhaust
      emissions. This translates into lost power, poor fuel economy
      and air pollution.
      The O2 sensor is constantly in a state of transition between high
      and low voltage. Manfucturers call this crossing of the 0.45
      volt mark O2 cross counts. The higher the number of O2 cross
      counts, the better the sensor and other parts of the computer
      control system are working. It is important to remember that the
      O2 sensor is comparing the amount of Oxygen inside and outside
      the engine. If the outside of the sensor should become blocked,
      or coated with oil, sound insulation, undercoating or antifreeze,
      (among other things), this comparison is not possible.

    34. 04-29-2004 10:54 PM #34
      Well if people can clamp the map sensor why cant you clamp the o2 sensor with a diode? Seems to me that if you used a multimeter and tested the values the sensor was outputting you could put a diode of the wattage inline and solve this problem completely.

    35. 04-30-2004 08:44 AM #35
      Quote, originally posted by ghettlo »
      ... you could put a diode of the wattage inline and solve this problem completely.

      one of the main purposes of this mod is that is non-permanent, so you do not have to hack up your O2 sensor harness. it is just another alternative... it's all about the options. [IMG]http://**********************/smile/emthup.gif[/IMG]

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