Theory of Operation
WARNING: This page is about a different car, the 2011 RAM Pickup. However, it is still accessible from the selected car via links, so may be relevant.
When the engine runs in DeNOx mode, the Powertrain Control Module (PCM) monitors how much post injction fuel has been injected. Once it exceeds a threshold, the measured O2 Sensor 1/1 is supposed to be lower than a threshold. If after a sufficient amount of post fuel is injected, the O2 Sensor 1/1 value is still high for a calibrated time, the fail counter is incremented by 1. If it is not, it is decremented by 1. If the counter has been incremented or decremented for a calibrated number of times, and the counter value is greater than a calibrated value, this fault is set.