Thousands of technicians have faced the
challenge of bringing a particularly stubborn application
into compliance for NOx emissions. High NOx readings are
a clear sign of performance issues somewhere in the system.
The question is, where?
Among the potential causes of non-compliant NOx readings
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.