Does the downstream o2 sensor effect anything? - Honda Civic Forum

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Does the downstream o2 sensor effect anything?

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Old 12-05-2017
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Does the downstream o2 sensor effect anything?

I have heard from a friend’s tuner that the ECU will freak out and run rich if the post cat o2 sensor is missing. Is that true? I thought that the entire purpose of the downstream o2 sensor was to make sure that the cat is working? I didn’t think that not having a downstream o2 sensor would effect the car at all besides causing an engine light. Who is right?

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Old 12-05-2017
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Re: Does the downstream o2 sensor effect anything?

Who is right?
Maybe neither.
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Old 12-05-2017
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Re: Does the downstream o2 sensor effect anything?

Originally Posted by ezone View Post
Maybe neither.
What do you mean?
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Old 12-05-2017
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Re: Does the downstream o2 sensor effect anything?

I have heard from a friend’s tuner that the ECU will freak out and run rich if the post cat o2 sensor is missing. Is that true?
Missing, or inop/dead/numb/no-fouler installed?

Depends on how the engineers programmed the original control unit and the strategies they intended it to use.
One design may be affected minimally if a sensor is numb, another design may be greatly affected.



If a TUNER is doing it and they can't make the processor ignore the rear sensor input....what does that say?


I thought that the entire purpose of the downstream o2 sensor was to make sure that the cat is working? I didn’t think that not having a downstream o2 sensor would effect the car at all besides causing an engine light.
Cat monitoring may have been an original design intent while OBD2 was being dreamed up and hammered out, but that's not the only thing going on.

Engineers design these things with emissions control FIRST, the idea that the first function of the engine and all fuel controls is a life support system for the catalytic converter. The engine just happens to pull the car around in the process.


Rear sensor can have a lot of influence over fuel trim values and strategies. The newer the car, the more important it is.

Even a cat fault code can cause a large drop in fuel economy on some cars.
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Re: Does the downstream o2 sensor effect anything?

Originally Posted by ezone View Post
Missing, or inop/dead/numb/no-fouler installed?

Depends on how the engineers programmed the original control unit and the strategies they intended it to use.
One design may be affected minimally if a sensor is numb, another design may be greatly affected.



If a TUNER is doing it and they can't make the processor ignore the rear sensor input....what does that say?


Cat monitoring may have been an original design intent while OBD2 was being dreamed up and hammered out, but that's not the only thing going on.

Engineers design these things with emissions control FIRST, the idea that the first function of the engine and all fuel controls is a life support system for the catalytic converter. The engine just happens to pull the car around in the process.


Rear sensor can have a lot of influence over fuel trim values and strategies. The newer the car, the more important it is.

Even a cat fault code can cause a large drop in fuel economy on some cars.
My 2005 Civic is missing the downstream o2 sensor and has no cat. If I get a junkyard downstream o2 sensor and install a defouler, will the car run better, worse, or no different?
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Old 12-05-2017
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Re: Does the downstream o2 sensor effect anything?

Originally Posted by D17VTECPOWER View Post
My 2005 Civic is missing the downstream o2 sensor and has no cat. If I get a junkyard downstream o2 sensor and install a defouler, will the car run better, worse, or no different?
If it works fine that way for you and you're ok with the CEL on all the time, DFWI.

Probably no real noticeable difference in how it runs, but the ECM or PCM will continually attempt to run the cat monitor self testing as long as there is no fault code for either O2 sensor....
Do you know how that cat testing is done?
By manipulating fuel controls.
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Old 12-05-2017
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Re: Does the downstream o2 sensor effect anything?

Originally Posted by D17VTECPOWER View Post
My 2005 Civic is missing the downstream o2 sensor and has no cat. If I get a junkyard downstream o2 sensor and install a defouler, will the car run better, worse, or no different?
I tried the same setup with a 6th gen I owned a few years ago. I used two spark-plug defoulers piggy-backed as described in this thread: https://dodgeforum.com/forum/2nd-gen...t-removed.html .Had car for 10k miles afterwards and the CEL never came on again for secondary o2 code.

Didn't seem effect performance in any way.
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