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Do downstream O2 ever get poisoned? #5136751
06/17/19 03:51 PM
06/17/19 03:51 PM
Joined: Aug 2002
Posts: 15,264
Silicon Valley
PandaBear Offline OP
PandaBear  Offline OP

Joined: Aug 2002
Posts: 15,264
Silicon Valley
Just a curiosity question. Since poisoning is one of the possible reason why CAT becomes ineffective, will it always be poisoned (i.e. by coolant, silicone, phosphate in oil) before the downstream O2 ever did? Is there any possibility that if both of them are poisoned, or if the CAT is poisoned long enough to have the downstream O2 also poisoned, that you need to replace both?

I assume it is impossible for the downstream O2 to be poisoned with the cat still functioning well?


"You keep asking questions PandaBear and you'll end up a vegetarian like my wife" - Camu Mahubah
Re: Do downstream O2 ever get poisoned? [Re: PandaBear] #5136800
06/17/19 04:33 PM
06/17/19 04:33 PM
Joined: Sep 2015
Posts: 1,945
USA
mk378 Offline
mk378  Offline

Joined: Sep 2015
Posts: 1,945
USA
A code P0420 is thrown because the downstream reading varies too rapidly, and more or less in sync with the upstream, suggesting the cat is not doing anything to change exhaust composition. This is one of the few codes that with considerable certainty tells you exactly which part is bad. If the downstream sensor were bad it would not vary, and you'll get a sensor code instead. This is pretty easy to confirm on a scanner that shows realtime graphs.

Re: Do downstream O2 ever get poisoned? [Re: PandaBear] #5136844
06/17/19 05:19 PM
06/17/19 05:19 PM
Joined: Oct 2015
Posts: 332
Washington
Dylan1303 Offline
Dylan1303  Offline

Joined: Oct 2015
Posts: 332
Washington
The second o2 is only there to see if the cat is working. The primary one is the most important. I had a slight exhaust leak on my downpipe off the turbo and my fuel trims where -18. It was stupid rich for a month until i figured out it was the vband clamp. The air from the exhaust leak made the primary o2 very upset lol. Also im tuned and the reading from my second o2 are shut off.


12 cruze rs 1.4t 6spd pp 5w30 tuned on 22psi
13 equinox 2.4 awd pp 5w30
Re: Do downstream O2 ever get poisoned? [Re: PandaBear] #5136903
06/17/19 06:17 PM
06/17/19 06:17 PM
Joined: Aug 2002
Posts: 15,264
Silicon Valley
PandaBear Offline OP
PandaBear  Offline OP

Joined: Aug 2002
Posts: 15,264
Silicon Valley
Will running with a bad cat too long get the downstream O2 foul / poison too?


"You keep asking questions PandaBear and you'll end up a vegetarian like my wife" - Camu Mahubah
Re: Do downstream O2 ever get poisoned? [Re: PandaBear] #5136909
06/17/19 06:24 PM
06/17/19 06:24 PM
Joined: Mar 2008
Posts: 8,501
USA
slacktide_bitog Offline
slacktide_bitog  Offline

Joined: Mar 2008
Posts: 8,501
USA
Yes, the downstream o2 can get poisoned. I had to replace one when the cat was fine smile

Re: Do downstream O2 ever get poisoned? [Re: PandaBear] #5137077
06/17/19 09:43 PM
06/17/19 09:43 PM
Joined: Aug 2010
Posts: 1,747
pa
benjy Offline
benjy  Offline

Joined: Aug 2010
Posts: 1,747
pa
also replaced the cheaper downstream sensor. depending on how bad cat a cat is clogged your engine won't run, worse case overheating + possibly catching fire!! the smell is said to be like rotton eggs!

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