Theory Of Operation

WARNING: This page is about a different car, the 2017 RAM 2500 HD. 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 DEF 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 with the SCR Catalyst) converts the NOx to harmless nitrogen and water. Excessive fuel or oil entering the exhaust system due to a failed engine component or Fuel Injector can cause the temperature to rise and can result in damage to the SCR Catalyst. The DEF Dosing Control Unit (DCU) is used to control the operation and monitoring of the SCR system.

The DEF DCU communicates with the Powertrain Control Module (PCM) over Data Link circuits. The SCR system is equipped with two NOx Sensors. The NOx Sensors consist of a probe mounted to the exhaust pipe which is hard-wired to a module mounted further away to isolate it from the exhaust heat. These NOx Sensors are used to monitor the efficiency of the SCR Catalyst and the rest of the DEF system. The NOx Sensor probe and module are calibrated to each other and must be replaced as an assembly. The two NOx Sensors are not interchangeable. The NOx Sensor modules are smart devices that communicate with the PCM over Data Link circuits. The NOx Sensor Modules perform their own internal diagnostics and report malfunctions back to the PCM.