NOx emissions though, are generally a result of high combustion temperatures. The EGR valve (or a cam timing equivalent) is typically what helps reduce the NOx emissions.

You might post up a copy of the report itself as well. Sometimes, other emissions leves can be higher than it should but still within passing range. Short of multiple smog device failures, seeing the broader range of emissions might help to more easily diagnose the source of the problem.

all i know is my NOx gases fail by over 500%!!!!

good thing i don't have to get tested!

the bpt is located right above the egr and looks like a disk with 2 hoses on top
the b esure the hardline from the egr to the exhs manifold isnt clogged or leaking.

between each check the nox should be going down if faulty. we chased down a noix probl;em on a hb for 2 yrs after switching the bpt valve 3 times.

a mod of the bpt valve took nox from 1300 to under 700 in i hr.
cats have an effect but try testing b4 replacing.
remeber as nox goes down, co2 often goes up.. its a balance