System Description

Front HO2Sis located on exhaust manifold before warm-up catalytic converter. To obtain a high purification rate for CO, HC, and NOx components of exhaust gas, a three way catalytic converter is used, but for most efficient use of three way catalytic converter, air ratio must be precisely controlled so that it is always close to stoichiometric air-fuel ratio. Heated Oxygen Sensor (HO2S) has the characteristic whereby its output voltage change suddenly in the vicinity of stoichiometric air-fuel ratio. This characteristic is used to detect oxygen concentration in exhaust gas and provide feedback to computer to control of air-fuel ratio. When air-fuel ratio becomes LEAN, Oxygen concentration in exhaust increases and HO2S informs PCM/ECM of LEAN condition (small electromotive force: zero volts). When air-fuel ratio is RICHER that stoichiometric air-fuel ratio oxygen concentration in exhaust gas is reduced and HO2S informs PCM/ECM of RICH condition (large electromotive force: 1 volt). PCM/ECM judges by voltage from HO2S whether air-fuel ratio is LEAN or RICH and controls injection time accordingly. However, if malfunction of HO2S causes output of abnormal voltage, PCM/ECM is unable to perform accurate air-fuel ratio control. HO2Ss include a heater which heats Zirconia element. Heater is controlled by PCM/ECM. When intake air volume is low (temperature of exhaust gas is low) current flows to heater to heat sensor for accurate oxygen concentration detection.