Oxygen Sensor: Description and Operation
OXYGEN SENSOR (O2S) - PCM INPUTThe Oxygen Sensors (O2S) are attached to, and protrude into the vehicle exhaust system. Depending on the emission package, the vehicle may use a total of either 1 or 2 sensor(s).
For these engines: If equipped with the CNG fuel system, only 1 sensor is used. It is referred to as an upstream or 1/1 sensor. The sensor is located in the exhaust downpipe before the main catalytic convertor.
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 sensors receive their fresh oxygen (outside air) supply through the wire harness. This is why it is important to never solder an 02 sensor connector, or pack the connector with grease.
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.
Oxygen Sensor Heaters/Heater Relays: The heating element(s) within the sensor(s) will be supplied voltage from the ASD relay.
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 maintain 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.
For these engines: Only one sensor (upstream 1/1) is used with the CNG emissions package. The PCM monitors the oxygen content of this sensor, and will fine-tune the air-fuel ratio. Main catalytic convertor efficiency is not calculated with this emissions package.
Engines equipped with a downstream sensor(s) will monitor catalytic convertor efficiency. If efficiency is below emission standards, the Malfunction Indicator Lamp (MIL) will be illuminated and a Diagnostic Trouble Code (DTC) will be set. Refer to Monitored Systems in Emission Control Systems for additional information.