Compass 4WD L4-2.4L VIN W (2007)
Oxygen Sensor: Description and Operation
Operation
OPERATION
Separate upstream and downstream grounds are used on the NGC vehicles (4 Cyl.).
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 and illuminates the malfunction
indicator lamp (MIL).
The O2 sensors produce a constant 2.5 volts on NGC vehicles, 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 produce a low voltage. When there is a lesser amount
of oxygen present (caused by a rich air/fuel mixture, which 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 O2 sensor's 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 NGC Controller has a common ground for the heater in the O2S. 12 volts is supplied to the heater in the O2S by the NGC controller. Both the
upstream and downstream O2 sensors for NGC are pulse width modulation (PWM). NOTE: When replacing an O2 Sensor, the PCM RAM memory
must be cleared, either by disconnecting the PCM C-1 connector or momentarily disconnecting the Battery negative terminal. The NGC learns the
characteristics of each O2 heater element and these old values should be cleared when installing a new O2 sensor. The customer may experience
driveability issues if this is not performed.
UPSTREAM OXYGEN SENSOR
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 2.5 to 3.5 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 2.5 volt. When there is a lesser amount of oxygen present
(rich air-fuel mixture) the sensor produces a voltage as high as 3.5 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 heated oxygen sensor input is used to detect catalytic convertor deterioration. As the convertor deteriorates, the input from the
downstream sensor begins to match the upstream sensor input except for a slight time delay. By comparing the downstream heated oxygen sensor input
to the input from the upstream sensor, the PCM calculates catalytic convertor efficiency. This is also used to establish the upstream O2 goal voltage
(switching point).
