Circuit Description
The ECM provides a pulse width modulated control circuit to adjust current through the heater. The A/F ratio sensor heater circuit uses a relay on the B+ side of the circuit. See Fig 1. To obtain a high purification rate for the CO, HC and NOx components of the exhaust gas, a 3-way catalytic converter is used, but for the most efficient use of the 3-way catalytic converter, 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 gas and provides feedback to the computer for control of the air-fuel ratio. When the air-fuel ratio becomes LEAN, the oxygen concentration in the exhaust increases and the oxygen sensor informs the ECM of the LEAN condition. 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. The 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 ECM is unable to perform accurate air-fuel ratio control. The heated oxygen sensors include 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. See Fig 2 and Fig 3 . Possible causes are:
- Open or short in heater circuit of heated oxygen sensor.
- Faulty heated oxygen sensor heater.
- Faulty EFI main relay.
- Faulty ECM.