P20EE - SCR NOx Catalyst Efficiency Below Threshold Bank 1
- Faulty NOx Sensor
- Diesel Exhaust Fluid
- Faulty Selective Catalyst Reduction (SCR) Catalyst
- Engine Light ON (or Service Engine Soon Warning Light)
The Selective Catalyst Reduction (SCR) catalyst is monitored to ensure it is capable of NOx conversion. The concentration of NOx upstream of the SCR is calculated based on a model. NOx concentration downstream of the SCR is measured with a NOx sensor. Using these concentrations, the cumulative efficiency of the SCR catalyst is calculated and compared to a threshold. If the cumulative efficiency is below this threshold for a sufficient period of time, a fault will be indicated.
The reductant, Diesel Exhaust Fluid (DEF), which is used as part of the SCR catalyst reaction, is monitored to ensure the tank is not refilled with an improper reductant. Upon detection of a refill event, the monitor is activated. After the SCR Catalyst Efficiency Monitor has completed and the SCR has been determined to be functional, the efficiency monitor continues to calculate the cumulative efficiency of the system. Each subsequent value for cumulative efficiency is included in two filtering routines, one for short term efficiency and the other for long term efficiency. If the difference between the two filtered efficiencies becomes greater than a threshold, a fault is indicated. the short term efficiency needs to be less than 0.20 and the delta between short and long term efficiency needs to be greater than 0.10.