The development of NOx sensor started in the 1990s. Commercial sensors were first introduced in the early 2000s on lean-burn, stratified charge gasoline passenger cars with NOx adsorbers, followed by diesel cars with NOx adsorbers and light- and heavy-duty diesel engines with urea-SCR aftertreatment.
The first generation of NOx sensors was developed by NTK, also known as NGK/NTK or NGK Spark Plug .
Another area of NOx sensor application has opened with the introduction of NOx adsorber catalysts on light-duty diesel engines. Some of the first applications included the Toyota DPNR system, launched in 2003, and the diesel engine Renault Espace model. The technology was widely adopted on diesel cars—primarily in Europe, but also in the US and other markets—including models from Volkswagen, BMW, and Daimler. These vehicles were typically equipped with a NOx sensor after the NOx storage catalytic converter.
The most recent area of NOx sensor application are urea-SCR systems for light- and heavy-duty diesel engines. To satisfy various OBD (on-board diagnostics) requirements, SCR systems typically use a NOx sensor downstream of the SCR catalyst. If excessive NOx or ammonia concentrations exist at the SCR outlet, an OBD malfunction will be triggered, as NOx sensors are sensitive to both gases. Depending on the SCR control strategy, another NOx sensor may be installed in front of the SCR catalytic converter. If two sensors are installed, the conversion rate of the SCR catalytic converter can be easily determined.
Further development of NOx sensors is driven by future heavy-duty engine emission standards such as those being proposed by CARB and the US EPA for 2027. The NOx limits may be lowered to values as low as 0.015 g/bhp-hr, while the durability and useful life requirements could be extended up to 850,000 miles (1,360,000 km) and 18 years. Improved sensor performance would not only be required for potential changes to OBD thresholds but also for in-use emissions monitoring that is being proposed as an alternative to the more conventional durability demonstrations. NOx sensor technology would need to develop further to be able to monitor emissions at low NOx levels, over the whole duty cycle of heavy-duty vehicle operations, and over their entire useful life.