Theory Of Operation
There are two Nitrogen Oxide (NOx) Sensors in the Exhaust System used to monitor Selective Catalyst Reduction (SCR) system efficiency. The SCR system includes, but is not limited to, the SCR Catalyst, Diesel Exhaust Fluid (DEF) injector, and the Exhaust Gas Temperature (EGT) Sensors. Physical issues or circuit DTCs with any of DEF, Particulate Matter (PM), or EGT Sensor(s) should be addressed before continuing with any Performance or Efficiency related DTC.
The Upstream NOx Sensor 1/1 is mounted on the Exhaust between the Turbocharger and the SCR Catalyst. The Downstream NOx Sensor 1/2 is mounted after the SCR Catalyst on the Exhaust. Both Sensors a have harness and module that are not interchangeable with each other or service separately.
During coast-downs or deceleration events the both NOx Sensor values on the scan tool should read 0 ppm or very close to 0 ppm. The engine does not produce any NOx during coast-downs as there is zero fueling occurring during coast-downs. During idle and when the Exhaust Temperature Senors are all reading about 150°C (302°F) both NOx Sensor ppm values should be similar, but NOx Sensor 2 should never be higher than NOx Sensor 1.