In direct contact with untreated, undiluted exhaust gases, the resistance of the thin film semiconductor tin oxide NOx sensor (TFS sensor) shows strong dependence on engine air-to-fuel ratio (A/F). The peak resistance of the sensor occurs around 17–18 A/F. From frequency response tests, the sensor response time constant is estimated to be ~1.1 seconds. A chemiluminiscent NOx analyzer (CLA) is used to measure the NO and NOx concentrations in the exhaust gases, while the NO2 content is estimated from [NOx] - [NO] measurements. Peak concentrations of NO and NO2 occur at 15 A/F and 17–18 A/F, respectively. The NO2 peak position is very close to the sensor resistance peak position. While EGR drastically reduces the NO content, it shows a smaller effect on the sensor resistance, especially around the stoichiometric condition.