Follow this link for information about NOx failures. Your Acura Legend may very well need an EGR cleaning, but you also want to make sure the engine is in proper fuel control, and is not running lean.
Your Acura Legend will create high NOx when the engine combustion chamber temps reach over 2500f. A lean fuel condition, high compression, defective EGR system will cause this to happen. If after you've ensured these 3 conditions are not the problem, you may suspect a damaged CAT. No not replace your Acura Legend's CAT with out making sure the engine is running right.
