A method (1000, 1100) of testing performance of a gas sensor assembly, comprising: applying a testing gas (1010, 1110) adjacent a gas sensor assembly; obtaining a first reading value of concentration of testing gas (1012, 1112); storing the first reading value (1018, 1118); obtaining a second gas sensor assembly reading value of concentration of testing gas (1022, 1122); determining a rate-of-rise value of the first and second reading values of the gas sensor assembly based on a difference of the first and second reading values relative to a testing time interval (1020, 1120) therebetween; determining if a gas sensor assembly passing condition exists based on a comparison of the rate-of-rise value of the first and second reading values to at least a first predefined rate-of-rise value of the gas sensor assembly; wherein before applying the testing gas to obtain the first reading value, the method further comprises a preliminary step of determining if a testing threshold value is obtained (1006, 1106), which testing threshold value is a minimum concentration value for which the gas sensor assembly is validated, where if said testing threshold value is not obtained during said preliminary step, the method proceeds to applying the testing gas and where if said testing threshold value is obtained during said preliminary step, the method further includes another preliminary step of clearing air adjacent the gas sensor assembly so that testing gas concentration does not at least reach the threshold value; and wherein the passing condition exists if the rate-of-rise value of the first and second reading values at least reaches the first predefined rate-of-rise value and does not at least reach a second predefined rate-of-rise value. applying a testing gas (1010, 1110) adjacent a gas sensor assembly; obtaining a first reading value of concentration of testing gas (1012, 1112); storing the first reading value (1018, 1118); obtaining a second gas sensor assembly reading value of concentration of testing gas (1022, 1122); determining a rate-of-rise value of the first and second reading values of the gas sensor assembly based on a difference of the first and second reading values relative to a testing time interval (1020, 1120) therebetween; determining if a gas sensor assembly passing condition exists based on a comparison of the rate-of-rise value of the first and second reading values to at least a first predefined rate-of-rise value of the gas sensor assembly; wherein before applying the testing gas to obtain the first reading value, the method further comprises a preliminary step of determining if a testing threshold value is obtained (1006, 1106), which testing threshold value is a minimum concentration value for which the gas sensor assembly is validated, where if said testing threshold value is not obtained during said preliminary step, the method proceeds to applying the testing gas and where if said testing threshold value is obtained during said preliminary step, the method further includes another preliminary step of clearing air adjacent the gas sensor assembly so that testing gas concentration does not at least reach the threshold value; and wherein the passing condition exists if the rate-of-rise value of the first and second reading values at least reaches the first predefined rate-of-rise value and does not at least reach a second predefined rate-of-rise value.