Circuit Description

WARNING: This page is about a different car, the 2003 Toyota RAV4. However, it is still accessible from the selected car via links, so may be relevant.

To obtain a high purification rate of the CO, HC and NOx components of the exhaust gas, a three-way catalytic converter (TWC) is used. For the most efficient use of the TWC, the air-fuel ratio must be precisely controlled so that it is always close to the stoichiometric air-fuel ratio. The oxygen sensor has the characteristic whereby its output voltage changes suddenly in the vicinity of the stoichiometric air-fuel ratio. This is used to detect the oxygen concentration in the exhaust gas and provide feedback for the computer to control the air-fuel ratio. When the air-fuel ratio becomes LEAN, the oxygen concentration in the exhaust gas increases and the oxygen sensor informs the ECM of the LEAN condition (small electromotive force: less than 0.45 volt). When the air-fuel ratio is RICHER than the stoichiometric air-fuel ratio, the oxygen concentration in the exhaust gas is reduced and the oxygen sensor informs the ECM of the RICH condition (large electromotive force: greater than 0.45 volt). The engine ECM judges by the electromotive force from the oxygen sensor whether the air-fuel ratio is RICH or LEAN and controls the injection time accordingly. However, if malfunction of the oxygen sensor causes output of abnormal electromotive force, the engine ECM is unable to perform accurate air-fuel ratio control. The main heated oxygen sensors include a heater which heats the zirconia element. The heater is controlled by the engine ECM. When the intake air volume is low (the temperature of the exhaust gas is low), the current flows to the heater to heat the sensor for the accurate oxygen concentration detection. Heated oxygen sensors are located on exhaust manifold below catalytic converters. See Figure.