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Discussion Starter · #1 ·
Finally... it has arrived!
Non-permanent mechanical solution to the rear O2 CEL (P0420)

CEL free for 12,000 miles and counting! (as of 5/23/05)
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 & *****
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! http://****************.com/smile/emthup.gif

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
 

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Re: Solution to the rear O2 CEL (kdub790)

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 http://****************.com/smile/emthup.gif


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

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Discussion Starter · #3 ·
Re: Solution to the rear O2 CEL (jeller)

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.
 

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Re: Solution to the rear O2 CEL (kdub790)

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 http://****************.com/smile/emthup.gif
 

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Re: Solution to the rear O2 CEL (kdub790)

Interesting. My guess is the metal in the "spark plug arrestor " works as a heat sink around the o2 sensor, keeping it happy.
 

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Discussion Starter · #7 ·
Re: Solution to the rear O2 CEL (VDUBNDizzy)

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.
 

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Re: Solution to the rear O2 CEL (kdub790)

But you're implying that less heat will keep the code from popping. But it's the opposite that should be true.
 

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Re: Solution to the rear O2 CEL (VDUBNDizzy)

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.
 

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Re: Solution to the rear O2 CEL (VDUBNDizzy)

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.
 

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Re: Solution to the rear O2 CEL (slugII)

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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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.
 

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Discussion Starter · #13 ·
Re: Solution to the rear O2 CEL (Power5)

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
 

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Re: Solution to the rear O2 CEL (kdub790)

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!!!
 

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Discussion Starter · #15 ·
Re: Solution to the rear O2 CEL (slugII)

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

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Discussion Starter · #16 ·
Re: Solution to the rear O2 CEL (SlvrBllt)

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.
 

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Re: Solution to the rear O2 CEL (kdub790)

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
 

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Re: Solution to the rear O2 CEL (kdub790)

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...
 

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Discussion Starter · #19 ·
Re: Solution to the rear O2 CEL (jeller)

the base is not drilled, the one the sensor goes in has to be drilled for obvious reasons.
 

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Re: Solution to the rear O2 CEL (kdub790)

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