In order to obtain a high exhaust gas purification rate and reduce the CO, CO, NOx, and NOx in the exhaust gas, the EFI car must use a three-way catalytic converter. However, in order to use the catalytic converter effectively, the air-fuel ratio must be precisely controlled so that it always approaches the stoichiometric air-fuel ratio. The catalytic converter is usually installed between the exhaust manifold and the muffler. The oxygen sensor has a characteristic that its output voltage has abrupt changes near the theoretical air-fuel ratio (14.7:1). This characteristic 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 high, the concentration of oxygen in the exhaust gas increases and the oxygen sensor notifies the ECU of the state of the mixture lean (small electromotive force: 0 volts). When the air-fuel ratio is lower than the stoichiometric air-fuel ratio, the oxygen concentration in the exhaust gas decreases, and the state of the oxygen sensor (large electromotive force: 1 volt) informs the (ECU) computer.