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VW CC 2010 2.0L CBFA I installed an O2 sensor spacer on my downstream O2 sensor and it seemed to be working for a day. One day later, after the car had cooled down it would rough idle and stall when i started it for the first time in the morning. Removing the spacer fixed this.

I have read everywhere that downstream O2 sensors aren't used by the ECU at all and are only used for emissions. How is this possible?

See below image I installed spacer on #3 bung VW CC 2010 2.0L CBFA 3C0254506EX

Here is the spacer i used from amazon. https://www.amazon.com/dp/B00B1U6I54/ref=cm_sw_em_r_mt_dp_U_Cey9CbDH3SGW1

O2 Sensor spacer

Here is some logging I did for the sensors.

Not shown is short and long term fuel trim which were both steady -1% to 1%

Logging

  • Why did you fit a spacer? – Solar Mike Jun 3 at 18:44
  • Hi Mike, I had a P0420 code and had replaced all 3 oxygen sensors on my car. Some people have managed to get rid of that cat inefficiency with a spacer. When I bought the car 10,000 miles ago we had installed a new universal rear cat, not knowing that there was another one near the turbo. Front Cat is not sold separately and the part 3C0254506EX comes with both converters from vw but is $1,000. – Salvador Martinez Jun 3 at 18:53
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    Post cat o2 sensors can be used for final fuel trim adjustment as well as a way to verify catalyst operation. You might want to do some data logging to see what’s going on with your fuel trims and o2 sensors. – Ben Jun 4 at 2:17
  • What codes are you getting now? – GdD Jun 4 at 8:21
  • @GdD I am only getting p0420 – Salvador Martinez Jun 4 at 23:11
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Well, they ARE used by the ECU, but only to test the efficiency of the cat converter. They don't control air/fuel mixture tho. That's controlled by the upstream O2 sensors. So you've got some other problem going on.

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    Why would removing the spacer fix the issue then? – Salvador Martinez Jun 3 at 18:54
  • Do you have some sort of evidence for this in this particular make/model? I ask because, this is what I thought too, but apparently in the OP's case, the spacer actually made a difference in the engine running, so it seems to suggest that it might not be the case? – Kitsunemimi Jun 3 at 20:02
  • One thing about my car is that it has 3 oxygen sensors. One on the first cat, then one pre the second cat and of course then the downstream o2 sensor post second cat. I installed it on the post second cat bung. See this image it's labeled #3 i.imgur.com/9lt1CZa.png – Salvador Martinez Jun 3 at 20:30
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I have read everywhere that downstream O2 sensors aren't used by the ECU at all and are only used for emissions. How is this possible?

There do exist fuel-injection control systems which actively employ the post-cat O2 sensors for adjusting the air-fuel ratio (AFR). The topic was discussed to death on the E39 M5 forum that I used to frequent, and was the primary reason why removing the catalytic converter on the M5 had an awful impact on performance (and emissions). You can see more information about this topic in this Q&A:

Is the after catalyst oxygen sensor used for engine mixture control?

... One day later, after the car had cooled down it would rough idle and stall when i started it for the first time in the morning. Removing the spacer fixed this.

This doesn't surprise me. Adding the spacer introduces a dead-spot where exhaust gas doesn't flow across the O2 sensor as it would when the O2 sensor is installed in its proper location.

The effect of this is that when the AFR of the exhaust gas changes (changing load, RPM, etc.) the AFR of "trapped" exhaust in the deadheaded region does not reflect what is being felt in the engine. If the difference between the true and deadhead AFR is significant enough the engine can experience misfires and a lack of responsiveness.

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Oxygen sensors need to be in the flow of the exhaust to get a correct reading. Maybe your spacer took the sensor too far out of the flow?

  • Yes, but why would the car stall from downstream spacer? – Salvador Martinez Jun 3 at 19:35
  • Well, my question is "why did you put a spacer in?" – Huesmann Jun 3 at 19:39
  • I'm not sure if you read my reply to Mike. I had a P0420 code which signifies a Cat Converter inefficiency. I then replaced all 3 oxygen sensors in hope that it would solve the problem. It did not. When I purchased the car 10,000 miles ago we had installed a new universal cat and the P0420 went away for a couple of months, but we didn't know that the car had 2 converters. The first one is next to the turbo and is not replaceable other than buying the whole assembly from VW 3C0254506EX which is $1000. Spacers have given many people success in getting rid of the P0420. – Salvador Martinez Jun 3 at 19:48
  • Then I guess my next Q is "what do you mean by an O2 sensor spacer?" To me, that is indicative of a spacer that is threaded into the O2 sensor bung, with male threads on one end and female on the other. Or possibly just a plan thin unthreaded spacer, though I'm not sure how much thread engagement you'd get with that kind of spacer. – Huesmann Jun 3 at 19:51
  • I purchased the spacer from amazon. Here is the link. It has male and female threads and fit really good. amazon.com/dp/B00B1U6I54/ref=cm_sw_em_r_mt_dp_U_Cey9CbDH3SGW1 – Salvador Martinez Jun 3 at 20:22
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Although technically there are ways this could happen, these are very very specific and likely would not happen so....

No, it is not possible. The ECU monitors your O2 sensor to know how it needs to adjust its fueling, this can be done in different ways depending on what type of O2 sensor and how the ECU manages its fuel.

However, as long as the Oxygen sensor still gets a good ground through the exhaust (i.e. the Spacer isn't some special material that doesn't conduct electricity) the ECU won't be able to tell.

  • The spacer will not insulate the sensor - as the threads of the sensor are still in contact... – Solar Mike Jun 11 at 5:57
  • @SolarMike, that's what I meant by as long as the Spacer isn't a material that doesn't conduct (which is essentially never going to be the case), as you say, if the spacer is metal it will ground through the threads and be fine. – H. Daun Jun 12 at 1:46

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