Circuit Description
The front Heated Oxygen (HO2) sensor monitors the oxygen concentration in the exhaust gas. For optimum three-way catalytic converter operation, the air-fuel mixture must be maintained as close as possible to the stoichiometric ratio. The HO2 sensor output voltage changes dramatically in the vicinity of the stoichiometric ratio. By making adjustments in accordance to these signal voltages, the ECM adjusts the fuel injection time so that the air-fuel ratio remains close to stoichiometric levels.
If the oxygen concentration in the exhaust gas increases, the air-fuel ratio is lean and the HO2 sensor output voltage to the ECM drops below 0.45 V. If the oxygen concentration in the exhaust gas decreases, the air-fuel ratio is rich and the HO2 sensor output voltage to the ECM increases to above 0.45 V.
HINT:
The output voltage of the HO2 sensor and the short-term fuel trim value can be read using a hand-held tester or an OBD II scan tool.