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Possible Causes of High Oxides of Nitrogen (NOx) Emissions
In order to pinpoint the cause of excessive NOx emissions, the following systems (if applicable) and possible defects will need to be checked, usually in this order:
Over-Advanced Ignition Timing
Older vehicles have ignition timing systems that are adjustable and, unfortunately, anything that is adjustable is also mal-adjustable.
Fortunately, ignition timing is usually quick and easy to check. For more details, see Ignition System Testing.
Inadequate Exhaust Gas Recirculation (EGR) Flow
Operation of the Exhaust Gas Recirculation (EGR) system should be checked completely. To do this there are three areas that the technician must check:
the EGR valve
the EGR exhaust passage(s)
the EGR control system
For more information on these procedures see Exhaust Gas Recirculation (EGR) System Testing.
Lean Air-Fuel Ratio
The technician should check to ensure that there is not an excessively lean mixture being burned. Depending on the vehicle, its fuel system, and its emissions control systems, this may involve simply looking at the VIR, measuring engine-out emission levels, and O2 sensor testing.
For detailed info regarding the diagnosis of fuel delivery systems, see Fuel System Testing.
Defective Three-Way Catalytic Converter
Your technician should test the catalyst by performing a converter efficiency test (see Catalytic Converter Testing). Note that this only applies if the vehicle is equipped with a three-way catalytic converter (some 1980-87 vehicles and all 1988 and newer vehicles have three-way catalytic converters).
Combustion Chamber Deposits
If the above possibilities have been conclusively checked and no problems found, it is reasonable to conclude that combustion chamber deposits are contributing to excessive NOx emissions. For additional information see Combustion Chamber Deposits.
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