Heated Oxygen Sensor
The upstream Heated Oxygen Sensor (HO2S) threads into the outlet flange of the exhaust manifold. See Fig 1 and Fig 2 . The downstream heated oxygen sensor threads into the system depending on emission package. On models with Federal emission package, the HO2S is mounted after the catalytic converter. See Fig 3. On models with Low Emission Vehicle (LEV) package, the HO2S is mounted mid catalytic converter. On models with, Ultra Low Emission Vehicle (ULEV) package, HO2S is mounted between catalytic converters. See Fig 4.
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 Powertrain Control Module (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). Normally HO2S produce voltages from 0 to 1 volt, depending upon the oxygen content of the exhaust gas in the exhaust manifold, this voltage is offset by a constant 2.5 volts on Next Generation Controller (NGC) vehicles. 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 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.
NGC has a common ground for the heater in the O2 sensor. Twelve volts is supplied to the heater in the O2 sensor by the NGC controller. Both the upstream and downstream O2 sensors for NGC are Pulse Width Modulation (PWM).
- Upstream Oxygen Sensor
The input from the upstream HO2S tells the Powertrain Control Module (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, (this voltage is offset by a constant 2.5 volts on NGC vehicles). 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).