Oxygen Sensor: Description and Operation

Fig.4 Oxygen Sensor 1/1 Upstream And 1/2 Downstream:

HEATED OXYGEN SENSORS-PCM INPUT
The upstream oxygen sensor threads into the outlet flange of the exhaust manifold. The downstream oxygen sensor threads into the side of the catalytic converter.

The O2 sensors produce voltages 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 sensors produce a voltage below 0.45 volts. When there is a lesser amount present (rich air/fuel mixture) it produces a voltage above 0.45 volts. By monitoring the oxygen content and converting it to electrical voltage, the sensors act as a rich-lean switch.

The oxygen 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. Upstream O2S (California emission equipped) are Pulse Width Modulated (PWM). They have a start-up delay to remove moisture from the heater element on cold start. This start-up delay may cause the DRB O2 Heater actuator test to be denied. The ground circuit is controlled by the PCM. This allows the PCM to duty cycle the heating element for the upstream O2S

In Closed Loop operation the PCM monitors the O2 sensor 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 Automatic Shutdown (ASD) relay supplies battery voltage to both the upstream and downstream heated oxygen sensors, The oxygen sensors are equipped with a heating element. The heating elements reduce the time required for the sensors to reach operating temperature.

Upstream Oxygen Sensor 1/1
The input from the upstream heated oxygen 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 input switches 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 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 inputs. In Open Loop, the PCM adjusts injector pulse width based on preprogrammed (fixed) values and inputs from other sensors.

Downstream Oxygen Sensor 1/2
The Downstream O2 Sensor has two functions. One function is measuring catalyst efficiency. This is an OBD II requirement. The oxygen content of the exhaust gasses has significantly less fluctuation than at the inlet if the converter is working properly. The PCM compares upstream and Downstream 02 Sensor switch rates under specific operating conditions to determine if the catalyst is functioning proper~.

The other function is a downstream fuel control. The upstream 02 goal varies within the window of operation of the O2 Sensor. In the past the goal was a preprogrammed fixed value based upon where it believed the catalyst operated most efficiently.

While the Upstream O2 Sensor input is used to maintain the 14.7:1 air/fuel ratio, variations in engines, exhaust systems and catalytic converters may cause this ratio to not be the most ideal for a particular catalyst and engine. To help maintain the catalyst operating at maximum efficiency, the PCM will fine tune the air/fuel ratio entering the catalyst based upon the oxygen content leaving the catalyst. This is accomplished by modifying the Upstream O2 Sensor voltage goal.

If the exhaust leaving the catalyst has too much oxygen (lean) the PCM increases the upstream 02 goal which increases fuel in the mixture causing less oxygen to be left over. Conversely, if the oxygen con- tent leaving the catalyst has is too little oxygen (rich) the PCM decreases the upstream O2 goal down which removes fuel from the mixture causing more oxygen to be left over. This function only occurs during downstream closed loop mode operation.