A test method for testing a piezoelectric gas sensor device, the method comprising: providing a self-test circuit module, STCM, adapted to couple to an output port of a piezoelectric Unit Under Test, PZ-UUT,; receiving an output signal from the PZ-UUT in response to the PZ-UUT being stimulated by an excitation signal, the excitation signal being obtained by (a) injecting excitation energy to the PZ-UUT during a first time-interval, and (b) supplying substantially no excitation energy to the PZ-UUT during a second time-interval immediately subsequent to the first time-interval; determining, with the STCM, the amplitude of a frequency content (155A, 155B) of the received output signal during the second time-interval; comparing, with the STCM, the determined amplitude of the frequency content (155A, 155B) at a predetermined comparison time during the second time-interval to a predetermined threshold amplitude; if the determined amplitude of the frequency content (155A, 155B) is less than the predetermined threshold amplitude, then generating a status signal indicative of a fault in the PZ-UUT; wherein the value of the predetermined threshold amplitude accepts good gas sensor devices with an acceptable time-domain response signal which follows a damped natural frequency of the piezoelectric element and fails bad gas sensor devices with an unacceptable time-domain response signal which drops to substantially zero within a time determined by a damping function having a time constant of less than about 50% of the second time-interval providing a self-test circuit module, STCM, adapted to couple to an output port of a piezoelectric Unit Under Test, PZ-UUT,; receiving an output signal from the PZ-UUT in response to the PZ-UUT being stimulated by an excitation signal, the excitation signal being obtained by (a) injecting excitation energy to the PZ-UUT during a first time-interval, and (b) supplying substantially no excitation energy to the PZ-UUT during a second time-interval immediately subsequent to the first time-interval; (a) injecting excitation energy to the PZ-UUT during a first time-interval, and (b) supplying substantially no excitation energy to the PZ-UUT during a second time-interval immediately subsequent to the first time-interval; determining, with the STCM, the amplitude of a frequency content (155A, 155B) of the received output signal during the second time-interval; comparing, with the STCM, the determined amplitude of the frequency content (155A, 155B) at a predetermined comparison time during the second time-interval to a predetermined threshold amplitude; if the determined amplitude of the frequency content (155A, 155B) is less than the predetermined threshold amplitude, then generating a status signal indicative of a fault in the PZ-UUT; wherein the value of the predetermined threshold amplitude accepts good gas sensor devices with an acceptable time-domain response signal which follows a damped natural frequency of the piezoelectric element and fails bad gas sensor devices with an unacceptable time-domain response signal which drops to substantially zero within a time determined by a damping function having a time constant of less than about 50% of the second time-interval