Electrochemical NOx sensors for high temperature applications were prepared coupling pellets of yttria-stabilized zirconia (YSZ), an oxygen ion conductor, with a thick film of a semiconducting oxide (WO3) as an auxiliary phase. The obtained sensors were exposed to the same atmosphere without the use of reference air. Electromotive force (EMF), polarization curves and amperometric measurements were performed in air and at different concentrations of NO2 in air, at selected temperatures. Stable EMF values and fast response times were obtained. The sensors showed non-linear polarization curves. The role played by the electrolyte/electrode interface was determined by electrochemical impedance spectroscopy (EIS) tests: exposure to NO2 did not affect the bulk but only the electrolyte/electrode interface.