The diffusion mechanism of a zirconia-based amperometric NOx sensor was examined by studying the effect of sensor temperature on sensor output. The structure and the exact dimensions of a production automotive NOx sensor were first measured using x-ray tomography. A simplified heat transfer model was employed and validated to estimate the sensor temperature based on sensor heater power. The sensor temperature was then changed by changing the sensor heater power to evaluate different diffusion mechanisms. Normal multi-component diffusion mechanism, Knudsen diffusion mechanism and a combination of both mechanisms (Normal and Knudsen diffusion) were evaluated at different sensor temperatures. The normal multi-component diffusion mechanism was experimentally found to be the dominant diffusion mechanism that affects the diffusive flow through the sensor diffusion barriers. A sensor model was developed based on this dominant diffusion mechanism to predict NOx concentration. Finally, the sensor model output for NOx was validated with the experiments at different Diesel engine operating conditions with different species concentrations.