The NOx/O2 sensor has been developed based on a simple tubular design. It is a potentiometric sensor that gives out two sensor voltages: one for the O2 concentration, the other for the NOx concentration. The NOx sensor has been extensively tested in synthetic gases and in real exhaust gases. It gives a good response towards NOx and NO2 for O2 concentrations between 0.5 and 20.8%. The sensor can be used to detect NOx in the range from 10-1500 ppm. Typical gas lab measurements to check the sensitivity and cross-sensitivity are shown. Succesfull sensor testings before and after a lean NOx trap (LNT) on a GDI engine are performed. The sensors are also tested on an exhaust gas simulator to check their sensitivity towards NO in more realistic circumstances. The fact that this is a potentiometric sensor has important advantages, e.g. the simple design, the better sensitivity towards low NOx concentrations, the good long term stability and the easy signal processing because of the voltage signals. This sensor is ideally suited for the monitoring and control of different kinds of NOx catalysts and/or for the measurement of raw NOx emissions in exhaust systems.