In this paper, thermal stability and reliability of a semiconductor gas sensor based on alumina substrates are analyzed by applying certain voltage of a heater inside the gas sensor. The heater normal operating voltage and temperature are 4.612V and 350 degrees Celsius respectively. The stability is measured while maintaining 4.612V and 4.629V for 830 hours. To measure the degree of a crack in the heater, a temperature acceleration experiment is performed for 7 days by applying a voltage of 4.94V or higher, and the value at high temperature is measured and compared. Initially, the certain resistance of the heater is 53 ohms. As a higher voltage is applied, the temperature increases. As a result, the resistance gradually changes. When the sensor reaches the limit point, the resistance decreases and saturation occurs at 40 ohms. The stability experiment results confirm that the inaccuracy increases above 375 degrees Celsius. Through the temperature acceleration experiments, the degree of the crack is measured to the point where the resistance change is 5%. As a result, the degree of damage at different temperature is estimated.