a contaminant sensor is provided which has an ultrasonic guided wave transducer that is bonded to a wall in which the ultrasonic energy is transmitted . a detector reflector is provided at a selected distance from the transducer , which will reflect signals that are affected or changed by contaminants collecting on an exposed surface of the wall . a self test reflector is positioned closely adjacent to the transducer and provides a reflected signal that is substantially unaffected by contaminants on the exposed surface of the wall , but which reflects signals only when the transducer assembly is functioning properly , and is properly bonded to the wall . the signal that is reflected by the self test reflector changes when the bond loosens . the receiver is activated right after transmission ceases to determine if the transmitter is operating .