One of the most effective technologies in decreasing large-scale NO_x emission produced by diesel engine vehicles is Urea-SCR (selective catalytic reduction) system. In order to prevent inducing excessive ammonia to the environment, an NH₃ sensor is required at the exit of this system. In this study, highly selective ammonia sensors were developed to detect ammonia emissions from automotive exhaust. The sensors were fabricated with 8-YSZ electrolyte, a platinum reference electrode and a working electrode of Au-V₂O₅ (mass ratio 85/15), screen-printed on an alumina substrate. A platinum resistor was printed at the backside of the support to control the sensor temperature. The measured sensor’ response (ΔV) is the potential difference between reference and working electrodes. Figure 1 shows the responses of two identical sensors to 100 ppm CO, NO₂, NO and 20 ppm of NH₃ at four different temperatures. It can be seen that the sensors respond to all gases at lower temperatures while by increasing temperature to 600°C, the selectivity to NH₃ is greatly improved. The selectivity of sensors was also confirmed by testing other possible interfering gases: no responses were observed for 20ppm of hydrogen and 100ppm of a hydrocarbon mixture. The stability of such sensors was studied at 550°C and 600°C. Since the sensors show no long-term stability at 600°C (electrode degradation), but remain stable at 550 °C, investigations were made to decrease the working temperature while maintaining selectivity. After testing different mass percentages of V₂O₅ in working electrode, we observed that by increasing this value to 50%, the working temperature of selective ammonia sensors could be decreased to 550°C with stable responses. Further investigations will be performed in order to gain deeper insight in sensing mechanism of V₂O₅ based working electrodes, which governs the sensor’s performance.