Selective Catalyst Reduction (SCR) NOx Sensors and Modules

The NOx sensor comprises an integrated heater and sensor control module. The sensor is used to measure the O2 and the NOx concentration in the exhaust gas of a diesel engine. The sensor and the control module are replaced as an assembly.

The NOx sensor is an evolution of the wide band Heated Oxygen sensor (HO2S). The sensor element is constructed from special ceramics and contains two oxygen density detecting chambers that work together to determine the NOx concentration in the exhaust gas. The NOx sensor control module sends a message via a private CAN bus to the PCM for monitoring the effectiveness of the SCR System.

The functionality of the SCR catalytic converter is monitored by the NOx sensor(s) and NOx modeling software in the PCM.

On EU6 vehicles the NOx mass entering the SCR catalytic converter is measured by a software model within the PCM.

On NAS vehicles, the mass entering the SCR catalytic converter is measured by a pre SCR catalytic converter NOx sensor.

On EU6 and NAS vehicles, the NOx mass emitted from the SCR catalytic converter is measured by a post SCR catalytic converter NOx sensor.

Values are measured over a certain period of time and the actual NOx reduction is compared to a minimum expected criteria calculated within the PCM.

Failure of a NOx sensor or the NOx emissions fail to reach a predetermined threshold, the PCM will record a fault, messages will be displayed to the driver in the Instrument cluster (IC) and the Malfunction Indicator Lamp (MIL) will be illuminated.

It is a legislative requirement to prevent the engine from starting if a fault occurs in the SCR system. As long as the fault is still present the IC message will provide a distance countdown to customer after which engine start will not be possible.