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 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.

The DEF Dosing Control Unit (DCU) is used to control the operation and monitoring of the DEF System, and communicates with the Powertrain Control Module (PCM) over the J1939 Data Link CAN Bus. The SCR system is equipped with two NOx sensors that are used to monitor the efficiency of the SCR Catalyst and DEF system. The NOx Sensor Probes and NOx Sensor Modules are calibrated to each other and must be replaced as an assembly. The two NOx Sensor Probes and Module assemblies are not interchangeable. The NOx Sensor Modules are smart devices that perform their own internal diagnostics and report malfunctions and information to the PCM over the J1939 Data Link CAN Bus.

This diagnostic will fail when the PCM detects the rate of NOx conversion efficiency across the SCR catalyst is deteriorating faster than that of a degraded SCR catalyst.