Oxygen Sensor: Description and Operation

HEATED OXYGEN SENSOR #1 (upstream)

Heated Oxygen Sensor:

PURPOSE
The Heated Exhaust Gas Oxygen Sensor #1 (HO2S) detects the presence of oxygen in the exhaust system and produces a variable voltage according to the amount of oxygen present.

When operating in closed loop the Powertrain Control Module (PCM) monitors this input and correspondingly changes the duration of the fuel injection pulse width to achieve an air/fuel ratio of 14.7:1.

CONSTRUCTION
The sensor is a zirconium dioxide ceramic thimble with a platinum surface electrode. The sensor is threaded into the exhaust manifold with a sampling tip extending into the exhaust stream.

Heated Oxygen Sensor Operation:

OPERATION
The zirconium dioxide in the sensor is electrically sensitive to differences in the amount of oxygen present between two sources. The outside of the sensor is vented to the atmosphere while the inside is exposed to the exhaust stream. The differing amounts of oxygen present between the two sources causes the sensor to generate a voltage.

A rich fuel mixture will result in a low level of oxygen present in the exhaust. This will produce a larger difference in oxygen levels sensed by the HO2S which will result in a higher voltage being produced.

A lean fuel mixture will result in a high level of oxygen present in the exhaust. This will produce a smaller difference in oxygen levels sensed by the HO2S which will result in a lower voltage being produced.

Normal operating range for the HO2S is 0.0 volts (lean condition) to 1.1 volts (rich condition).

The HO2S temperature must be above 600°F to operate properly. To minimize heat up time, a heating element is incorporated into the sensor.

HEATED OXYGEN SENSOR #2 or CATALYST MONITOR SENSOR (downstream)

PURPOSE
The Heated Exhaust Gas Oxygen Sensor #2 (HO2S) indicates catalyst efficiency based on oxygen storage capacity.

OPERATION
Closed loop fuel control is transferred from upstream to downstream HO2S. The switching frequency of the downstream HO2S output is measured. This actual measured output frequency is called the test frequency. The test frequency is an indication of the oxygen storage capacity of the catalytic converter. The oxygen storage capacity of a high efficiency will have a low switch ration and high HC efficiencies. As catalyst efficiency deteriorates, its ability to store oxygen declines and it will begin to have a higher switch ratio and low HC efficiencies.