Among the potential causes of non-compliant NOx readings are:
Malfunctioning or improperly adjusted EGR valve
Failed oxygen sensor
Leak in exhaust tubing upstream of converter
Excessive carbon deposits in combustion chamber
Improper spark advance
Blocked coolant passage
Overly lean air-fuel mixture
Damaged cold air duct
Failed or malfunctioning catalytic converter
Corroded or damaged engine sensor electrical connections
Please note that while a failed catalytic converter will contribute to high NOx readings, the failure in itself may have been caused by some other upstream performance problem. In all cases, it is crucial to identify the root cause of the problem before blaming the converter.
" I'm working on a 1992 3.1-liter Chevrolet that failed in the test lane. The results were very good for HC and CO—nearly zero—but it failed for NOx. The converter seems to be operating properly and the engine is running okay. What's causing the problem?"
Answer: Because this application, like many others, does not have an EGR adjustment, there's little the technician can do to "tweak" the engine's performance to bring it into compliance. It's clear the engine is running overly lean. A lean engine operating condition produces more NOx than usual, and the lean exhaust chemically interferes with the converter's ability to clean NOx (remember that excess oxygen is good for cleaning HC and CO, but bad for NOx).
There are dozens of potential causes of the high NOx readings, ranging from the relatively easy (detonation or failed oxygen sensor) to the extremely difficult and time-intensive (an improperly signaling MAP sensor). Because the engine is otherwise performing satisfactorily, the least expensive option may be to upgrade to a heavier duty converter such as a CleanAir "Premium" unit featuring increased catalyst for greater efficiency.