Heated Oxygen Sensor
The upstream Heated Oxygen Sensor (HO2S) threads into the outlet flange of the exhaust manifold and the downstream HO2S threads into the outlet pipe at the rear of the catalytic converter. See Fig 1 -Fig 3 .
A single sensor ground is used for all HO2S's. As vehicle accumulates mileage, the catalytic converter deteriorates. The deterioration results in a less efficient catalyst. To monitor catalytic converter deterioration, the fuel injection system uses two heated oxygen sensors. One sensor upstream of the catalytic converter and one downstream of the converter. The PCM compares the reading from the sensors to calculate the catalytic converter 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 and illuminates the Malfunction Indicator Light (MIL). The HO2S 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, 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 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. In closed loop operation, the PCM monitors the HO2S input (along with other inputs) and adjusts the injector pulse width accordingly. During open loop operation, the PCM ignores the HO2S 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 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
The input from the upstream HO2S 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
The downstream HO2S input is used to detect catalytic converter deterioration. As the converter deteriorates, the input from the downstream sensor begins to match the upstream sensor input except for a slight time delay. By comparing the downstream HO2S input to the input from the upstream sensor, the PCM calculates catalytic converter efficiency. Downstream HO2S input is also used to establish the upstream O2 goal voltage (switching point).