P0420 mysterious problem

[kolyani4]

Hello, we have a problem, that i cant figure why is that happens.

i have a Chevy Malibu 2015 with LTG engine.
i have a custom exhaust with High-Flow 3" Downpipe with 200 Cell Magnaflow Cat.
when the tune that i use is loaded there is no P0420 Code is appearing ( DTC reports not touched in a tune ), when i load a stock tune, the P0420 is coming after about 70km of highway cruising.
i want to understand why the thing is happening and if this is dangerous.
one thing that i think about is that somehow the AFR is messing and its get LEAN burn and Exhaust temp get up and heats up the cat and its burns things "better" hope i"m wrong.

i"m attaching stock and my tune file ( based on ATS diablo tune ).

[TCSS07]

I'm mobile atm but with having the exhaust mods and high flow cat - 99% of the time in my experience they will always set a p0420 because you now have increased the airflow and the o2 sensor is toggling to fast. Disabling the dtc is the only fix imo. You could try your luck and install some o2 foulers and see

just looked at the files and yeah i couldn't tell you except for maybe with the changes made to the tune by making the trims closer and everything a more in sync with the engine torque tables and what not the ecm isnt seeing a problem

[kolyani4]

disable the dtc is no a problem, but i want to understand why that thing is happens, there is no something in the tune that messed up, but i didnt want to blow engine or valves because of this, for now im trying to figure what excactly makes ecm to think that "cat are ok".

[kolyani4]

UPDATE:
Yesterday i loaded a stock tune with only change in Power Enrichment section.
today i was cruised a 200km and there is no DTC code appear. so for now i"m pretty sure that this is related to power enrichment, but why that is happens, im pretty sure that i even didnt entered in PE mode today, most of the road that i rode today was in Cruise control mode.

[TCSS07]

Maybe ride it out the way it is now or a few days and see if it the dtc pops back on

Just find it odd that changing pe settings would negate the p0420 test

[kolyani4]

Maybe ride it out the way it is now or a few days and see if it the dtc pops back on

Just find it odd that changing pe settings would negate the p0420 test

yes, i want to find what exactly in PE setting couse DTC not to popup. my guest that its a "PE torque" but not sure. i will update soon.
sound very odd, but there is only setting from a stock tune. i will try to come in another way, load a tuned tune with stock PE settings.

[Jeff7577]

Was your converter new when it set the P0420? Are you logging upstream and downstream sensors? How many times has it set P0420? I don't think the diagnostic will ever run with how your tune is setup.

[kolyani4]

Was your converter new when it set the P0420? Are you logging upstream and downstream sensors? How many times has it set P0420? I don't think the diagnostic will ever run with how your tune is setup.

Yea, the converter is new. im sure that it monitor the cat in this tune, because when the original cat was bad, its throws me a p0420. after i do a catted downpipe its runs well without any dtc. one day i decided to test a stock tune to compare a fuel consumption ( its almost same ), but when i drove about 100km after a tune on highway its throw me a cat dtc and continue to throw it every 70-100km . first thing that i thinked it that cat are failed. but when i loaded the tuned file and goes back to home, there is no cat dtc was.

why you think that the diagnostic will not run ?