Theory of Operation

WARNING: This page is about a different car, the 2011 RAM Cab & Chassis. However, it is still accessible from the selected car via links, so may be relevant.

Selective Catalytic Reduction (SCR) is a technology that uses a UREA based diesel exhaust fluid (DEF) and a catalytic converter to significantly reduce nitrous oxides (NOx) emissions. The system accomplishes this by injecting small quantities of diesel exhaust fluid (DEF) are injected into the exhaust system upstream of the catalyst, where it vaporizes and decomposes to form ammonia and carbon dioxide. The ammonia is the desired product which in conjunction to the SCR Catalyst, converts the NOx to harmless nitrogen and water. The Diesel Exhaust Fluid (DEF) Dosing Control Unit is used to control the operation and monitoring of the Diesel Exhaust Fluid system. The DEF Dosing Control Unit communicates with the Powertrain Control Module over the J1939 Data Link. The SCR system is equipped with two NOx Sensors and modules that are used to monitor the efficiency of the SCR Catalyst and diesel exhaust fluid system. The NOx Sensors and NOx Sensor Modules are calibrated to each other and must be replaced as an assembly. The two NOx Sensor and Module assemblies are not interchangeable. The NOx Sensor Modules are smart devices that communicate with the Powertrain Control Module (PCM) over the J1939 Data Link. The NOx Sensor Modules perform their own internal diagnostics and report malfunctions back to the PCM.