Patient is an '03 A6 2.7T with FTs, DPs with no pre-cats but main cats.  Cats are probably not premium grade if I'm honest.  Bosch EV14s, Ford 42 lb.  Rear O2s coded out per the wiki except for ESKONF.  Trims are ~+2.5.  I'm in Colorado where in the Denver area we do an IM240 at ~30 and ~55 mph.  The test is desinged to find problems and includes brief periods of moderate acceleration followed by float, followed by decel, followed by accel.  CO and HC are great, but getting some NOx spikes on accel after a steady-state or decel, 2.0 gm/mi vs 1.5 gm/mi limit.

Second time through, as a data point more than anything else (although I was hoping I'd pass, of course), I reduced timing in the load ranges tested (for my application, maybe 40% or less) to 12-15* from 35*-40* AND (and this is where I may have screwed up) using LAMFA i went to 0.95 lamda.  Well, I failed on CO 30 gm/mi vs limit of 15 gm/mi altough NOx was great (as one might expect.)

From what I know, good CO and HC (second test notwithstanding) is usually not the catalyst but generally lean operation.  My wideband shows that the O2 sensors (at least the one upstream of my wideband) are spot-on.

Thoughts? I know the rears are in some applications used in a strategy to clear O2 and prevent NOx spikes, which sounds like I need to reinstall the rears.  Problem with reinstalling is that then I give a big clue that pre-cats are not installed since there may or may not be a visual depending on how diligent the tech is doing the test.  KFVAKL?  Try LAMFA again, but 0.98 or 0.99?  Bite the bullet and put the stock exhaust back on?

Oh, and for those of you in CO, changes coming in 2015 include 7-year testing exemption (up from 4), 8-11 years old must pass OBDII for O2, O2 heaters, and cat efficiency (CEL must also be off) or IM240 will be run, 12 years and older is mandatory IM240.

Thanks for the response, Lee. 

I'm going to re-install the rear O2s since the federal OBDII database shows that they need to be there and with them coded out of course the ECU reports only the main O2 sensors as being present and also cannot report whether cat efficiency is within limits.  In Colorado as of January 1, 2015, from what I understand a mismatch between the database and what's installed triggers a mandatory visual inspection, whereas previously it was just 'advised' and if the car passed the IM240 often it didn't happen. 

As well, having the rear O2s will reactivate the Cat O2 purge strategy and hopefully reduce/eliminate the spikes on accel after coast which was the main reason for the bust in the first place.  I think if I reduce timing as I did the first time around I may get there.  If not, I spring for some new cats.  Hopefully if OBDII is happy and the car passes on the rolling road there'll be no visual.