System Description

NOTE: Front Heated Oxygen Sensor (HO2S) may also be referred to as Bank 1/Sensor 1 (1/1) HO2S or O2 Sensor (Up). Rear HO2S may also be referred to as Bank 1/Sensor 2 (1/2) HO2S or O2 sensor (Down).

Front Heated Oxygen Sensor (HO2S) is located on exhaust manifold and rear HO2S is located after warm-up catalytic converter. ECM uses HO2S signals to maintain the air fuel mixture at the ratio resulting in optimum fuel economy and reduced emissions. Amount of oxygen in exhaust gases indicates to front HO2S whether air fuel mixture being supplied to engine cylinders is rich or lean. Readings of rear HO2S are used to indicate efficiency of catalytic converter. ECM calculates catalytic converter efficiency by comparing rear HO2S signal to front HO2S signal.

A normal HO2S signal will constantly fluctuate above and below 500 mV, with front HO2S signal frequency of at least 5 Hz at 2500 RPM. Due to effect of catalytic converter, rear HO2S signal frequency will be lower than front HO2S signal frequency. If rear HO2S signal coincides with front HO2S signal a large percentage of the time, this indicates a loss in efficiency of catalytic converter or a malfunction within fuel system.