cat removal

[ChrisZang]

Originally Posted by bmwm750

However, as to causing problems with passing emissions, removal of the main cats will throw no codes, as the cat efficiency monitoring uses the oxygen sensors before and after the pre cats, so removing the secondary cats has no effect on this process. Now, if the smog tech knows enough, he will notice that you do not have secondary cats and could fail the visual. Removing the pre cats in the headers is a lot more work when trying to keep the DME happy.

Thanks for your quick response. I guess the answer is Yes and No.
My understanding is that the DME is driven by the data from the first set of O2 sensors (before the pre-cats). So removal of the pre or main cats does not affect the DME at all.
The 2nd set of O2 sensors is basically just verifying that the cats are working at all, meaning the DME tests if the exhaust gas going by the the 2nd set of O2 sensors is cleaner as the gases passing the 1st set of sensors. Therefore the trick is (if you do not want to relocate the 2nd set of O2 sensors after removing the pre-cats) to simply let them "breathe fresh air" by - well - having them hang in the air and all is fine.

So I'd guess I am right by saying that the removal of the 1st of cats will more likely let me pass smog (as opposed to the rmeoval of the 2nd set of cats), also as it's much harder to see during a visual inspection that these have been removed.