It has become commonplace to say that diesel vehicles are equipped with more and more sophisticated exhaust after-treatment technologies to meet increasingly stringent emissions limits. However, depending on the way they are used and maintained, recent diesel vehicles may produce pollutant rates which largely deviate from certification levels, even after a moderate mileage and despite of the on-board electronic control capacity increase. A quick pollution test at exhaust (new method of engine diagnosis) was performed on 168 diesel cars to determine if any engine maintenance was needed. A typology of the encountered engine defects or malfunctions is presented. Furthermore, emissions measurements according to the European specifications and along representative driving cycles were carried out on two diesel cars. Deviations from the original emission levels, associated with voluntary faults generated with defective injectors and closed EGR on a 'sane' vehicle, were assessed. Reversely, deviations are also analysed after repair and maintenance of a vehicle with multiple defects.