Amperometric NOx sensors are increasingly used in automotive industry to meet the stringent emission measurement regulations. These sensors measure O-2 and NOx concentration using two different sensing cells. In this work, a physics-based model was developed and then employed to predict the sensor output for oxygen as a function of sensor temperature and oxygen concentration. A temperature perturbation method was also developed based on the model to calibrate the sensor output with respect to oxygen concentration. The model accurately matched the experimental results for steady state and transient conditions. A two step sensor diagnostics procedure based on the sensor temperature perturbation method was then proposed. The first diagnostics step evaluates the sensor output to check if it is within the acceptable range. The second diagnosis step checks the plausibility of the sensor output based on the physics based model and temperature perturbation. A self-calibration procedure was also implemented inside the diagnostics procedure using temperature perturbation at engine-off. This self-recalibration only requires an external relative humidity measurement.