If your Chevy truck's CO percent was normal (between .1 and .5) then we'd
recommend you look into the EGR valve (exhaust gas recirculation system).
The EGR valve is responsible for lowering NOx by re-circulating a small
amount of exhaust gas (inert gas) back into the combustion chambers while
the engine is running. Nitrous Oxides (No) emissions are formed when
combustion chamber temperatures rise above 2500F. Presenting an inert gas
keeps the chambers cool and reduces NOx.
The EGR valve on your Chevy pickup should be inspected to ensure it's
working properly. The EGR valve should be able to open using manifold
vacuum. When the valve is opened at idle you should notice a dramatic drop
in RPMs. The EGR system over time collects carbon. In time it may become
clogged and stuck open or closed from carbon deposits.
Other problems that could cause high NOx if the EGR system is working
properly could be bad engine cooling, lean fuel condition (though a lean
fuel condition would be suspected if CO levels were super low 0.00 to 0.05)
and high mileage which could mean there is carbon build up on the valves and
piston heads increase compression and engine combustion temperatures.
