SENSOR-OXYGEN
DESCRIPTION
The upstream oxygen sensor threads into the outlet
flange of the exhaust manifold.
The downstream heated oxygen sensor threads into
the system depending on emission package.
OPERATION
A 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 cat-
alyst. 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 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 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 injec-
tor pulse width based on preprogrammed (fixed) values and inputs from other sensors.
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 pro-
PM
FUEL INJECTION
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