As with the oxygen sensor, the air-fuel ratio sensor detects the oxygen concentration in the exhaust gas. Conventional oxygen sensors are such that the output voltage tends to change drastically at the boundary of the theoretical air-fuel ratio. In comparison, the air-fuel ratio sensor applies a constant voltage to obtain a voltage that is nearly proportional to the oxygen concentration. This improves the air-fuel ratio detection accuracy. The output illustration shows an air-fuel ratio sensor displayed in a hand-held tester. A circuit that maintains a constant voltage on the AF+ and AF- terminals of the engine ECU is built in. Therefore, the output condition of the air-fuel ratio sensor cannot be detected by a voltmeter. Please use the hand-held tester. The output characteristics of the air-fuel ratio sensor make it possible to corrects as soon as there is a change in the air-fuel ratio, which makes the air-fuel ratio feed-back correction faster and more accurate. As with some oxygen sensors, the air-fuel ratio sensor also has a heater for maintaining detection performance when the exhaust temperature is low. However, the air-fuel ratio sensor heater requires much more current than the heaters in the oxygen sensors.