The Mass Air Flow Sensor on your 6.7L Cummins is mounted at the air filter box and is designed to transfer signals to the ECM, informing it of how much airflow is coming into the engine so that it can adjust fueling, timing, and other parameters. Numerous issues can arise causing the MAF Sensor to send improper signals, one of which is running a dirty air filter or if the sensor becomes contaminated with air filter oil caused from over oiling a cleanable filter. A multitude of issues can arise from improper air flow signals to include a rough idle and poor overall driving characteristics.