Oxygen Sensor: Description and Operation

PURPOSE
The Heated Oxygen Sensor (HO2S) mounted in the exhaust downpipe serves as the key input in an electronic fuel injection system. The sensor is used by the Engine Control Module (ECM) to determine the amount of oxygen present in the exhaust gas. The ECM uses this information to increase or decrease injector open time to bring the air/fuel ratio as close to Stoichiometric as possible.

The ECM monitors each sensor separately and makes fuel trim adjustments to each cylinder bank independent of the other.

OPERATION
Oxygen sensors operate efficiently only when warm. Sensors include heaters to help them reach operating temperatures quickly. This allows the sensors to provide accurate information to the ECM soon after start-up and the system to enter closed loop operation (based on sensor inputs) sooner. Closed loop operation helps provide an efficient fuel mixture and controls emissions when the engine is cold. Closed loop also guards against catalytic converter overheating from the introduction of too much fuel.

Diagnostic Trouble Codes (DTC) 44/45 are produced if either Oxygen sensor will not switch after the ECM has radically altered fueling on that bank of injectors. The test is performed when the engine is at normal operating temperature, Throttle Position (TP) sensor input is above 2 volts, and a road speed input is received.