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A digital signal representative of difference in pressure is received from a differential pressure transmitter. A noise signal is obtained by processing the signal through a band pass filter or otherwise to provide a filtered noise signal. Absolute values of the filtered noise signal are calculated and compared to one or more predetermined threshold values to determine if one or more impulse lines are plugged. A training mode is used to determine the thresholds which may be a function of flow rate and other flow conditions. (FIG.1)