SENSOR-OXYGEN
DESCRIPTION
The Oxygen Sensors (O2S) are attached to, and protrude into the vehicle exhaust system. Depending on the
engine or emission package, the vehicle may use a total of either 2 or 4 sensors.
On this emissions package, 4 sensors are used: 2 upstream (referred to as 1/1 and 2/1) and 2 downstream
(referred to as 1/2 and 2/2). With this emission package, the right upstream sensor (2/1) is located in the right
exhaust downpipe just before the mini-catalytic convertor. The left upstream sensor (1/1) is located in the left
exhaust downpipe just before the mini-catalytic convertor. The right downstream sensor (2/2) is located in the right
exhaust downpipe just after the mini-catalytic convertor, and before the main catalytic convertor. The left down-
stream sensor (1/2) is located in the left exhaust downpipe just after the mini-catalytic convertor, and before the
main catalytic convertor.
OPERATION
An O2 sensor is a galvanic battery that provides the PCM with a voltage signal (0-1 volt) inversely proportional to
the amount of oxygen in the exhaust. In other words, if the oxygen content is low, the voltage output is high; if the
oxygen content is high the output voltage is low. The PCM uses this information to adjust injector pulse-width to
achieve the 14.7–to–1 air/fuel ratio necessary for proper engine operation and to control emissions.
The O2 sensor must have a source of oxygen from outside of the exhaust stream for comparison. Current O2 sen-
sors receive their fresh oxygen (outside air) supply through the O2 sensor case housing.
Four wires (circuits) are used on each O2 sensor: a 12–volt feed circuit for the sensor heating element; a ground
circuit for the heater element; a low-noise sensor return circuit to the PCM, and an input circuit from the sensor back
to the PCM to detect sensor operation.
Four heated oxygen sensors are used. A separate oxygen sensor relay is used to supply voltage to the sensors
heating elements for only the 1/2 and 2/2 downstream sensors. Voltage for the other 2 sensor heating elements is
supplied directly from the Powertrain Control Module (PCM) through a Pulse Width Module (PWM) method.
Pulse Width Module (PWM): Voltage to the O2 sensor heating elements is supplied directly from the Powertrain
Control Module (PCM) through two separate Pulse Width Module (PWM) low side drivers. PWM is used on the 2
upstream sensors (1/1 and 2/1). The main objective for a PWM driver is to avoid overheating of the O2 sensor
heater element. With exhaust temperatures increasing with time and engine speed, it’s not required to have a full-
voltage duty-cycle on the O2 heater elements.
To avoid the large simultaneous current surge needed to operate all 4 sensors, power is delayed to the 2 down-
stream heater elements by the PCM for approximately 2 seconds.
Oxygen Sensor Heater Elements:
The O2 sensor uses a Positive Thermal Co-efficient (PTC) heater element. As temperature increases, resistance
increases. At ambient temperatures around 70°F, the resistance of the heating element is approximately 4.5 ohms.
As the sensor’s temperature increases, resistance in the heater element increases. This allows the heater to main-
tain the optimum operating temperature of approximately 930°-1100°F (500°-600° C). Although the sensors operate
the same, there are physical differences, due to the environment that they operate in, that keep them from being
interchangeable.
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 certain O2 sensor input(s) 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.
Two upstream sensors are used (1/1 and 2/1). The 1/1 sensor is the first sensor to receive exhaust gases from the
#1 cylinder. They provide an input voltage to the PCM. The input tells the PCM the oxygen content of the exhaust
gas. The PCM uses this information to fine tune fuel delivery to maintain the correct oxygen content at the down-
stream oxygen sensors. The PCM will change the air/fuel ratio until the upstream sensors input a voltage that the
PCM has determined will make the downstream sensors output (oxygen content) correct.
The upstream oxygen sensors also provide an input to determine mini-catalyst efficiency. Main catalytic convertor
efficiency is not calculated with this package.
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FUEL INJECTION - 3.7L GAS
KJ