The properties of titania are such that its resistance changes in accordance with the oxygen concentration of the exhaust gas. This resistance changes abruptly at the boundary between a lean and a rich theoretical air/fuel ratio, as shown in the graph. The resistance of titania also changes greatly in response to changes in temperature. A heater is, thus built into the laminated substrate to keep the temperature of the element constant.

This sensor is connected to the ECM as shown in the following circuit diagram. A 1.0 volt potential is supplied at all times to the 0" positive (+) terminal by the ECM. The ECM has a built-in comparator that compares the voltage drop at the Ox terminal (due to the change in resistance of the titania) to a reference voltage (0.45 volts). If the result shows that the Ox voltage is greater than 0.45 volts (that is, if the oxygen sensor resistance is low), the ECM judges that the air/fuel ratio is rich. If the 0, voltage is lower than 0.45 volts (oxygen sensor resistance high), it judges that the air/fuel ratio is lean.

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