Selective Catalyst Reduction (SCR) Components - EU5/6 And NAS Only
| ITEM | DESCRIPTION |
|---|---|
| 1 | SCR Nox sensor |
| 2 | SCR Nox sensor module |
| 3 | Post SCR soot sensor (NAS only) |
| 4 | Post SCR soot sensor module (NAS only) |
EU5/6 vehicles have a single post SCR Nox sensor module and a post SCR Nox sensor located in the exhaust system after the Selective Catalyst Reduction (SCR) catalytic converter.
NAS vehicles have a pre SCR Nox sensor module and pre SCR Nox sensor located in the exhaust system after the catalytic converter and Diesel Particulate Filter (DPF) assembly. A second post SCR Nox sensor and post SCR Nox sensor module is located in the exhaust system after the Selective Catalyst Reduction (SCR) catalytic converter.
NAS vehicles also have a post SCR soot sensor located in the exhaust system after the SCR catalytic converter, adjacent to the post SCR Nox sensor.
SCR Nox Sensors and Modules (EU6 and NAS)
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 sensorHO2S). 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 Controller Area Network (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 modelling 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 pre-determined 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.
Post SCR Soot Sensor (NAS only)
The post SCR soot sensor measures the soot particles in the exhaust gas to evaluate the functionality of the DPF. The soot sensor comprises a sensor element attached to a dedicated control module with a hardwired connection. The sensor and control module are replaced as an assembly.
The absorbed soot particles create conductive paths between electrode combs in the soot sensor element. The control module measures the resistance between the electrode combs and sends a message to the PCM via a private CAN bus to evaluate the performance of the DPF.
The control module regularly overheats the electrode combs to regenerate the sensor element.
Failure of the post SCR soot sensor or the NOx emissions fail to reach a pre-determined 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.