The oxygen sensor is to detect the oxygen content in the exhaust gas to determine the concentration of the mixture gas, and fed back to the ECU. In order to obtain a high exhaust gas purification rate and reduce the (CO) carbon monoxide, (HC) hydrocarbon and (NOx) nitrogen oxide components in the exhaust, EFI vehicles must use a three-way catalyst. But in order to be able to use the three-way catalytic converter effectively, the air-fuel ratio must be accurately controlled so that it is always close to the theoretical air-fuel ratio. The three-way catalytic converter is usually installed between the exhaust manifold and the muffler.
The oxygen sensor has a characteristic that its output voltage has a sudden change near the theoretical air-fuel ratio (14.7:1). This feature is used to detect the concentration of oxygen in the exhaust gas and feed it back to the computer to control the air-fuel ratio.
When the actual air-fuel ratio becomes higher, the concentration of oxygen in the exhaust gas increases and the oxygen sensor notifies the ECU of the lean state of the mixture (small electromotive force: 0 volts). When the air-fuel ratio is lower than the theoretical air-fuel ratio, the concentration of oxygen in the exhaust gas decreases, and the state of the oxygen sensor (large electromotive force: 1 volt) informs the (ECU) computer.