Circuit Description
To obtain a high purification rate for CO, HC and NOx components of exhaust gas, a 3-way catalytic converter is used, but 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.
Heated oxygen sensor output voltage changes suddenly in vicinity of stoichiometric air-fuel ratio. This characteristic is used to detect oxygen concentration in exhaust gas and provide ECM with feedback to control air-fuel ratio.
When air-fuel ratio becomes LEAN, oxygen concentration in exhaust 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.
DTCs P0031, P0037, P0051 and P0057 are set when heater current of 0.2 amps or less is present when heater operates with +B at greater than 10.5 volts and less than 11.5 volts (one trip detection logic) or heater current of 0.25 amps or less when heater operates with +B at greater than 11.5 volts (one trip detection logic). DTCs P0032, P0038, P0052 and P0058 are set when heater operates and heater current exceeds 2 amps (one trip detection logic). Possible causes are:
- Open or short in heater circuit of heated oxygen sensor.
- Faulty heated oxygen sensor heater.
- Faulty relay.
- Faulty ECM.