Theory Of Operation

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 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. Excessive fuel or oil entering the exhaust system due to a failed engine component or Fuel Injector can cause the temperature to rise and will result in damage to the SCR Catalyst. 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.