This paper presents a self-diagnostic and sensor validation procedure that performs in-situ monitoring of the operational status of piezoelectric (PZT) sensors and actuators used in structural health monitoring (SHM) applications. The sensor/actuator self-diagnostic procedure, where the sensors/actuators are confirmed to be functioning properly during operation, is a critical component to successfully complete the SHM process with large numbers of active sensors typically deployed in a structure. Both degradation of the mechanical/electrical properties of a PZT transducer and the bonding defects between a PZT patch and a host structure could be identified using the proposed procedure. First, the effects of bonding defects between a PZT patch and a host structure on high frequency SHM techniques, including Lamb wave propagations and impedance methods, have been experimentally investigated. It has been found that the effects are remarkable, modifying wave phase and amplitude, creating new wave modes, and changing measured impedance spectrum. These changes can lead to the false indications on structural conditions without an efficient sensor-diagnostic procedure. The feasibility of the proposed sensor diagnostics procedure was then demonstrated by analytical studies and experimental examples, where the functionality of surface-mounted piezoelectric sensors was continuously deteriorated. The proposed procedure can provide a metric that can be used to determine the sensor functionality over a long period of service time or after an extreme loading event. Further, the proposed procedure can be useful if one needs to check the operational status of a sensing network right after its installation.