NOx (Nitrogen Oxides)

NOx (mostly nitric oxide and nitrogen dioxide) forms in the hottest parts of combustion. It contributes to smog and acid rain and is harmful to breathe, which is why emission standards limit it strictly. Petrol engines reduce NOx in the three-way catalyst. Diesels, which run with excess air, need EGR, NOx storage catalysts or SCR with AdBlue.

NOx is a collective term for nitric oxide (NO) and nitrogen dioxide (NO2). Diesel engines produce more NOx than petrol engines because of their lean, hot combustion. Nitrogen dioxide irritates the lungs and contributes to smog and ground-level ozone, which is why limits for road vehicles have been tightened over the years. Real driving emissions (RDE) tests in Europe now also measure NOx on public roads, not only in the laboratory.

NOx sensors in the exhaust measure the concentration so the control unit can control EGR and AdBlue dosing and check that the system works. Engines reduce NOx with exhaust gas recirculation and, on most modern diesels, SCR systems with AdBlue or NOx storage catalysts.

NOx-related codes, such as SCR efficiency faults, often lead to warnings and a no-restart countdown on diesels, because the car would exceed its legal emission limits. NOx limits are a major part of Euro and EPA emission standards. A failed NOx sensor or SCR fault can light warning lamps and, on many cars, limit restarts until fixed.