Circuit Description
To obtain a high purification rate of CO, HC and NOx components of exhaust gas, a 3-way catalytic converter is used. For most efficient use of 3-way catalytic converter, air-fuel ratio must be precisely controlled so that it is always close to stoichiometric air-fuel ratio. Air/fuel ratio sensor provides output voltage (voltage value changes at inside of ECM only) which is approximately proportional to existing air-fuel ratio. Air/fuel ratio sensor output voltage is used to provide feedback for ECM to control air-fuel ratio. With air/fuel ratio sensor output, ECM can determine deviation amount from stoichiometric air/fuel ratio and control proper injection time immediately. If air/fuel ratio sensor is does not function, ECM is unable to perform accurate air-fuel ratio control. Air/fuel ratio sensor is equipped with a heater which heats zirconia element. Heater is controlled by 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. See Fig 1.
DTC is recorded when air/fuel ratio sensor heater is malfunctioning. ECM provides a pulse width modulated control circuit to adjust current through heater. Air/fuel ratio sensor heater circuit uses a relay on B+ side of circuit. See Fig 2. If DTC is set, there is heater current of less than .8 amp or more than 19.7 amps when heater operates (2 trip detection logic). Check for open or short in heater circuit of air/fuel ratio sensor. Check air/fuel ratio sensor heater, air/fuel ratio sensor heater relay or ECM.