Do downstream O2 ever get poisoned?

[PandaBear]

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?

 

[mk378]

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.

 

[Dylan1303]

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.

 

[PandaBear]

Will running with a bad cat too long get the downstream O2 foul / poison too?

 

[slacktide_bitog]

Yes, the downstream o2 can get poisoned. I had to replace one when the cat was fine

 

[benjy]

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!