In order to develop an intelligent flow sensor which can be validated by itself, a wavelet-transform-based new method for assessing blockage of the sensing line in a differential-pressure flow sensor is proposed in this paper. The wavelet transform, a powerful means of signal processing for analysing the characteristic of a signal if there is little prior knowledge about it, is used to decompose the sensor-output signal into some elementary building blocks. The energy distributions in these building blocks reveal the effects of blockage of the sensing line of the sensor and then are exploited to determine their existence. Experimental results show that the incipience of the sensing-line blockage can be detected by this method.