Laboratory studies were performed to examine the effect of NOx on the static switch point of current (1996 MY) production-type heated exhaust gas oxygen (HEGO) sensors. These sensors are used on vehicles for feedback control of the A/F. Several binary gas mixtures and simulated exhaust gas mixtures were used to measure the static response to NO and NO2 in the laboratory. The results of the laboratory studies indicate that most of these HEGO sensors completely equilibrate NO, but only partially equilibrate NO2. To validate this result, engine studies were performed to investigate whether the A/F control point shifts when the amount of EGR is changed. Results from both the laboratory and engine studies are presented.