Failed Emissions Test – High NOx

Q: My car failed the emission testing at the inspection station. The inspector said the engine exceeded the acceptable oxides of nitrogen. What could cause these high levels?

A: High NOx (oxides of nitrogen) levels can be caused by incorrect ignition timing (common cause on older vehicles), faulty EGR (Exhaust Gas Recirculation) system, engine running lean (air-fuel ratio), engine overheating, carbon deposit buildup in the combustion chambers, or a defective catalytic converter.