Operation

OPERATION

An O2 Sensor is a galvanic battery that provides the Powertrain Control Module (PCM) with a voltage signal (0-1 Volt) inversely proportional to the amount of oxygen in the exhaust. In other words if the oxygen content is low, the voltage output is high; if the oxygen content is high, the voltage output is low. The PCM uses this information to adjust injector pulse-width to achieve the 14.7 to 1 air/fuel ratio necessary for proper engine operation and to control emissions. The O2 Sensor must have a source of oxygen outside the exhaust stream for comparison. Current O2 Sensors receive their fresh oxygen (outside air) supply through the O2 Sensor case housing. Four wires (circuits) are used on each O2 Sensor: a 12-volt feed circuit for the sensor heating element, a ground circuit for the heater element, a low-noise sensor return circuit to the PCM and an input circuit from the sensor back to the PCM to detect sensor operation.

As vehicles accumulate mileage, the catalytic convertor deteriorates. The deterioration results in a less efficient catalyst. To monitor catalytic convertor deterioration, the fuel injection system uses two heated oxygen sensors. One sensor upstream of the catalytic convertor, one downstream of the convertor. The PCM compares the reading from the sensors to calculate the catalytic convertor oxygen storage capacity and converter efficiency. Also, the PCM uses the upstream heated oxygen sensor input when adjusting injector pulse width.

When the catalytic converter efficiency drops below emission standards, the PCM stores a Diagnostic Trouble Code (DTC) and illuminates the Malfunction Indicator Lamp (MIL).

The O2 Sensors produce voltages from 0 to 1 volt, depending upon the oxygen content of the exhaust gas. When a large amount of oxygen is present (caused by a lean air/fuel mixture, can be caused by misfire and exhaust leaks), the sensors produces a low voltage. When there is a lesser amount of oxygen present (caused by a rich air/fuel mixture, can be caused by internal engine problems) it produces a higher voltage. By monitoring the oxygen content and converting it to electrical voltage, the sensors act as a rich-lean switch.

The O2 Sensors are equipped with a heating element that keeps the sensors at proper operating temperature during all operating modes. Maintaining correct sensor temperature at all times allows the system to enter into closed loop operation sooner. Also, it allows the system to remain in closed loop operation during periods of extended idle.

In Closed Loop operation the PCM monitors the O2 Sensors input (along with other inputs) and adjusts the injector pulse width accordingly. During Open Loop operation the PCM ignores the O2 Sensor input. The PCM adjusts injector pulse width based on preprogrammed (fixed) values and inputs from other sensors.

The Engine Control Relay located in the Relay Control Module supplies battery voltage to both the upstream and downstream heated O2 Sensors. The O2 Sensors are equipped with a heating element. The heating elements reduce the time required for the sensors to reach operating temperature. The PCM uses pulse width modulation to control the ground side of the heater to regulate the temperature.

UPSTREAM OXYGEN SENSOR

The input from the upstream heated O2 Sensor tells the PCM the oxygen content of the exhaust gas. Based on this input, the PCM fine tunes the air-fuel ratio by adjusting injector pulse width.

The sensor output varies from 0 to 1 volt, depending upon the oxygen content of the exhaust gas in the exhaust manifold. When a large amount of oxygen is present (caused by a lean air-fuel mixture), the sensor produces voltage as low as 0.1 volt. When there is a lesser amount of oxygen present (rich air-fuel mixture) the sensor produces a voltage as high as 1.0 volt. By monitoring the oxygen content in the exhaust gases and comparing that to the oxygen content in ambient air and converting it to electrical voltage, the sensor acts as a rich-lean switch.

The heating element in the sensor provides heat to the sensor ceramic element. Heating the sensor allows the system to enter into closed loop operation sooner. Also, it allows the system to remain in closed loop operation during periods of extended idle.

In Closed Loop, the PCM adjusts injector pulse width based on the upstream heated oxygen sensor input along with other signals. In Open Loop, the PCM adjusts injector pulse width based on preprogrammed (fixed) values and inputs from other sensors.

DOWNSTREAM OXYGEN SENSOR

The downstream heated O2 Sensor signal is used to detect catalytic convertor deterioration. As the convertor deteriorates, the signal from the downstream sensor begins to match the upstream sensor signal except for a slight time delay. By comparing the downstream heated oxygen sensor signal to the signal from the upstream sensor, the PCM calculates catalytic convertor efficiency. This calculation is also used to establish the upstream O2 goal voltage (switching point).