Selective Catalyst Reduction Components - If Equipped
| ITEM | DESCRIPTION |
|---|---|
| 1 | Post Selective Catalyst Reduction (SCR) particulate matter sensor |
| 2 | Post SCR particulate matter sensor module |
| 3 | Pre SCR NOx sensor module |
| 4 | Pre SCR NOx sensor |
| 5 | Post SCR NOx sensor module |
| 6 | Post SCR NOx sensor |
SCR NOx Sensor and Module
The pre SCR NOx sensor is located in the exhaust pipe upstream of the SCR catalytic converter. The post SCR NOx sensor is located in the exhaust pipe downstream of the SCR catalytic converter. The SCR NOx sensor comprises a sensor element attached to a dedicated SCR NOx sensor module with a hardwired connection.
The SCR NOx sensor is used to measure the oxygen and the NOx concentration in the exhaust gas of a diesel engine. The SCR NOx sensor and the control module are replaced as an assembly.
The sensor element is constructed from special ceramics and contains 2 oxygen density detecting chambers that work together to determine the NOx concentration in the exhaust gas. The first chamber identifies the oxygen level in the exhaust gas. The second chamber is used to calculate the amount of NOx, by separating and pumping the nitrogen away from the oxygen.
The SCR NOx sensor control module sends a message via a private Controller Area Network (CAN) bus to the Powertrain Control Module (PCM) for monitoring the effectiveness of the SCR system.
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 record a Diagnostic Trouble Code (DTC). A message is displayed to the driver in the Instrument Cluster (IC) and the Malfunction Indicator Lamp (MIL) is illuminated.
On North American Specification (NAS) market vehicles it is a legislative requirement to prevent the engine from starting if a fault occurs in the SCR system for the last 800 km (500 mi). With a fault detected in the SCR system, the IC displays the count down before engine restart is prevented.
Particulate Matter Sensor and Module
The post SCR particulate matter sensor is located in the exhaust pipe downstream of the SCR catalytic converter. The particulate matter sensor comprises a sensor element attached to a dedicated control module with a hardwired connection. The sensor and the control module are replaced as an assembly.
The sensor element comprises electrode combs. The deposited soot particles alter the resistance between the electrode combs. The particulate matter sensor reports out the resistance to a threshold. Signals from the particulate matter sensor sent to the Powertrain Control Module (PCM) via a private Controller Area Network (CAN) bus.
The signals from the particulate matter sensor is used by the PCM to determine whether the Diesel Particulate Filter (DPF) is functioning.
Prior to measurement phase, the electrode combs must be cleaned from soot particles. The control module heats a ceramic element in the particulate matter sensor to a specified temperature to clean the soot particles from between the combs.