thanks for the reply.

Just before the test I replaced the two air idle hoses from the idle valve. Also replaced the hose off the top of the air cleaner housing and the breather hose off the head. Also changed many of the small rubber hose fittings / elbows. I has a slight vacuum leak at the intake manifold and tightened it up and got much better results.

The vehicle does not have an EGR system so it relies heavily on the CAT to reduce NO.

I believe the O2 sensor is 33 plus years old the vehicle has 233 k miles

I have noticed over the years that they have lowered the limits however from 2 years ago to todays test they are the same.

What does a CO2 of 13.3 and 14.3 tell me?

What does a O2 of 1.6 and 1.0 tell me?