The resistivity of titania (TiO2) ceramic depends upon the partial pressure of oxygen (PO2) in the atmosphere surrounding the ceramic. Because there is a functional relationship between the PO2 of the equilibrated exhaust gas and the air to fuel ratio (A/F) operating point of the internal combustion engine, the resistance of a TiO2 ceramic sensor, when temperature controlled, can be used to determine quantitatively the A/F. TiO2 sensors utilizing these principles have been built and found to work particularly well in the region rich of stoichiometry. Control of the A/F in the region rich of stoichiometry is desirable for engine/catalyst systems designed to meet low NOx emission levels.This paper reports on the design details and operating features of an experimental TiO2 sensor and its associated electronic controller. In addition, experimental results are reported for the sensor/controller combination operating in engine dynamometer tests and on vehicles with feedback-controlled, electronic fuel injected engines. These results indicate that feedback control using the TiO2 sensor represents a possible means of achieving the precision A/F control required to obtain efficient performance of vehicles using NOx catalysts.