f oil, moisture, or other foreign substances enter the instrumentation equipment air due to an equipment failure, they often accumulate in the positioner and can cause it to malfunction. The air circuit diagnostics provide a diagnostic algorithm for detecting the progression of the accumulation of foreign substances through a tendency comparison of the relationship between the control signal and nozzle back pressure in the positioner. The positioner identifies trends caused by accumulation of foreign substances in the nozzle flapper and clogging of the seat area and informs the user when to perform maintenance on the air circuits.