The intrusive test is the only one that I use, and it can be inconclusive if there is any measurable upstream oxygen. 70% is the basic rule of thumb, but even a good cat can fall on its face in the presence of upstream oxygen or very low CO.
NOx in - NOx out
------------------- * 100% = NOx reduction efficiency
“Never attribute to malice that which can be adequately explained by stupidity.”
thanks barry.. yeah that is the formula i use.. and i do understand that upstream o2 like an exhaust leak or lean condition will cause it to not work right, but i keep getting these cars that are not lean no leaks but high nox... example: while back i had an 05 g35 high nox cats at about 70% no leaks no lean condition (no check engine light btw d.a.d wont catch this one) also i have a 92 explorer no egr, no lean condition, same thing slightly high nox and it would seem 70% just isnt enough or am i missing something???
Lack of EGR due to defective EGR system causes NOx failure even with new cat.
on the other hand old tired engine with compression loss can gain some power with EGR deleted (and HC emissions will be lower (due to higher combustion temperature)