Circuit Description

WARNING: This page is about a different car, the 2003 Lexus RX 300. However, it is still accessible from the selected car via links, so may be relevant.

To obtain a high purification rate for the CO, HC and NOx components of the exhaust gas, a three-way catalytic converter is used. But for the most efficient use of the three-way catalytic converter, the air-fuel ratio must be precisely controlled so that it is always close to the stoichiometric air-fuel ratio. The A/F sensor has the characteristic that provides output voltage approximately proportional to the existing air-fuel ratio. The A/F sensor output voltage is used to provide feedback for the ECM to control the air-fuel ratio. By the A/F sensor output, the ECM can determine the deviation amount from the stoichiometric air-fuel ratio and control the proper injection time immediately. If the A/F sensor is malfunctioning, ECM is unable to perform accurate air-fuel ratio control. The A/F sensor is equipped with a heater which heats the zirconia element. The heater is controlled by the ECM. When the intake air volume is low (the temperature of the exhaust gas is low), current flows to the heater to heat the sensor for accurate oxygen concentration detection.

DTCs are detected when voltage output of A/F sensor remains at 3.8 volts or more, or 2.8 volts or less, during engine running after engine is warmed up (2 trip detection logic), voltage output of A/F sensor does not change from 3.30 volts during engine running after engine is warmed up (2 trip detection logic), or open or short in A/F sensor circuit (2 trip detection logic). Possible causes are:

Open or short in A/F sensor circuit.
A/F sensor (bank 1, 2 sensor 1).
Air induction system.
Fuel pressure.
Injector.
ECM.