Circuit/System Description

The heated oxygen sensor (HO2S) consists of an oxygen sensing cell, an oxygen pumping cell and a heater. The exhaust gas sample passes through a diffusion gap between the sensing cell and the pumping cell. The engine control module (ECM) supplies a signal voltage to the HO2S and uses this voltage as a reference to the amount of oxygen in the exhaust system. An electronic circuit within the ECM controls the pump current through the oxygen pumping cell in order to maintain a constant signal voltage. The ECM monitors the voltage variation on the signal circuit and attempts to keep the voltage constant by increasing or decreasing the amount of current flow or reversing the direction of the current flow to the pumping cell. By measuring the direction and amount of current required to maintain the signal voltage, the ECM can determine the concentration of oxygen in the exhaust. The signal voltage is displayed as a lambda value. When the system is lean, the oxygen level will be high and the lambda value will be high. When the system is rich, the oxygen level is low and the lambda value will be low. The ECM uses this information to correct the variations in the fuel system.