Colorado changed the NOx requirements in January 2021. This area of Colorado is now under CA IM240 requirements. So this is a 49 state MTX being held to CA standards. My last passing result in 2020 was 2.42 GPM and my failing result yesterday was 2.31. Searching the archives there seems to be nothing on this topic in about 15 years. The old threads suggest: new ECT, new O2 sensors, cleaning MAF (especially if K&N filter is used), adding a few gallons of alcohol, and as a last gasp cheat, unplug the SPOUT connector.
I have my emissions results back to 2016 when I bought the car. The old limit was 2.5000 GPM. In 2016 I had a 1.7812; 2018 2.1278; 2020 2.4204; 2022 2.3066. 2016 was stock Y-pipe, remainder was ShoSource equal length with CATs. In 2021 the limit dropped to 2.0000 from 2.5000 so the 2.31 yesterday would have passed until the last year. Before someone says it - the stock Y-pipe is long gone. It took up too much space and I never dreamed I'd need it again.
Cleaned the MAF last night and replaced the K&N with a paper filter. I have ordered the ECT and O2 sensors from NAPA, should be here today. Or should I be looking for a stock Y-pipe in the Boulder/Denver area? Any other new wisdom in the last 15 years for NOx reduction? I'll sell the car before retrofitting and EGR and changing computers.