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New types of potentiometric NOx sensors suitable for use on automobiles were developed by using stabilized zirconia as a base solid electrolyte. It was found that the sensor with sensing electrodes of metal oxides (CdMn2O4 or NiCr2O4) showed excellent response to NOx in the concentrations between 20 and 4000 ppm at temperatures higher than 600 °C. The electromotive force of the sensors was almost linear to the logarithm of the NOx concentrations with positive slope for NO2 and negative slope for NO. Especially, the sensor fitted with CdMn2Os gave excellent responses to NO at 600 °C, while the sensor fitted with NiCr2O4 showed high sensitive to NO2 at 650 °C. The sensors were insensitive to CO, CO2, CH4, C3H6, O2 and water vapor. The sensors were fabricated as planar types with the reference electrode exposed to the sample gas, so that the sensors were simple in structure and easy to manufacture.