To obtain the DTC codes you must have the door closed, the transmission in Park, the Odometer/Trip in the Trip position.
Turn the key to on (do not start) and off, 3 times, within 5 seconds, ending in the on position, the display will first display ------ then any current DTC codes followed by "Done" and yes the ECM/PCM does store the codes in its memory, however the odometer only displays the current status, which does change depending on driving style.
Keeping the Engine Brake on helps to ensure the variable vane moves and minimizes the effects of the soot build up.