To maximum the exhaust purification function of the engine with TWC (Three-Way Catalytic Converter), the air-fuel ratio must be kept within a narrow range around the theoretical air-fuel ratio. The oxygen sensor detects whether the oxygen concentration in the exhaust gas is richer or leaner than the theoretical air-fuel ratio. The sensor is mainly installed in the exhaust manifold, but the location and number that are installed differ depending on the engine. The oxygen sensor contains an element made of zirconium oxide (ZrO2), which is a type of ceramic. The inside and outside of this element is covered with a thin coating of platinum. The ambient air is guided into the inside of the sensor and the outside of the sensor is exposed to the exhaust gas. At high temperatures (400C [752F] and higher), the zirconium element generates a voltage as a result of the large difference between the oxygen concentrations on the inside and outside of the zirconium element. In addition, the platinum acts as a catalyst to cause a chemical reaction between the oxygen and carbon monoxide (CO) in the exhaust gas. Therefore, this reduces the amount of oxygen and increases sensor sensitivity. When the air-fuel mixture is lean, there is much oxygen in the exhaust gas so that there is only a little difference in the oxygen concentration between the inside and outside of the zirconium element. Therefore, the zirconium element will only generate a low voltage (nearly V). Conversely, when the air-fuel mixture is rich, there is almost no oxygen in the exhaust gas. For this reason, there is a large difference in the oxygen concentration between the inside and outside of the sensor so that the zirconium element generates a relatively large voltage (approx. 1 V). Based on the OX signal output by the sensor, the engine ECU increases or decreases the fuel injection volume so that the average air-fuel ratio is maintained at the theoretical air-fuel ratio. Some zirconium oxygen sensors have heaters to heat the zirconia element. This heater is also controlled by the engine ECU. When the amount of the intake air is low (in other words, when the exhaust gas temperature is low), current is sent to the heater to heat the sensor.