Circuit Description
To obtain a high purification rate for CO, HC and NOx components of exhaust gas, a three-way catalytic converter is used. However, to use three-way catalytic converter most efficiently, air-fuel ratio is precisely controlled so it becomes close to stoichiometric air-fuel ratio. Heated oxygen sensor output voltage changes suddenly when value reaches stoichiometric air-fuel ratio. This is used to detect oxygen concentration in exhaust gas and provides feedback to computer for control of air-fuel ratio. When air/fuel ratio becomes LEAN, oxygen concentration in exhaust gas increases and heated oxygen sensor informs ECM of LEAN condition (small electromotive force: less than 0.45 volt). When air/fuel ratio is RICHER than stoichiometric air/fuel ratio, oxygen concentration in exhaust gas is reduced and heated oxygen sensor informs ECM of RICH condition (large electromotive force: greater than 0.45 volt). ECM judges by electromotive force from heated oxygen sensor whether air-fuel ratio is RICH or LEAN and controls injection time accordingly. However, if malfunction of heated oxygen sensor causes output of abnormal electromotive force, ECM is unable to perform accurate air-fuel ratio control. Heated oxygen sensors include 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. 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, rear heater current of .2 amp or less or more than 2.35 amp when heater operates is detected (2 trip detection logic). Check for open or short in heater circuit of heated oxygen sensor. Also check heated oxygen sensor, E.F.I. ECU relay, E.F.I. ECU relay circuit, or ECM.