Oxygen Sensor: Description and Operation
HEATED OXYGEN SENSOR (O2S SENSOR) - PCM INPUTFig. 3 Heated Oxygen Sensor 1/1:
Fig. 4 Heated Oxygen Sensor 2/1:
The upstream oxygen sensor threads into the outlet flange of the exhaust manifold or.
Fig. 5 Heated Oxygen Sensor 1/2, 2/2:
The downstream heated oxygen sensor threads into the outlet pipe at the rear of the catalytic convertor.
The O2 Sensors are zirconium dioxide, four wire and heated. The heater uses two of the sensors' four wires. One is common ground, the other provides battery voltage to the heater from the ASD Relay. One of the remaining wires is a sensor signal input to the PCM. The fourth wire is signal ground. The O2 Sensors deliver a voltage signal (0 - 1 volt) to the PCM inversely proportional to the amount of oxygen in the exhaust. If the oxygen content is low, the output voltage is high; if the oxygen content is high, the output voltage low. The O2 Sensors must have a source of oxygen from outside the exhaust stream for comparison. O2 Sensors receive their fresh oxygen supply through the wire harness. This is why it is important to never solder an O2 Sensor connector or pack the connector with grease.
The Automatic Shutdown (ASD) relay supplies battery voltage to both the upstream and downstream heated oxygen sensors. The oxygen sensors are equipped with a heating element. The heating elements reduce the time required for the sensors to reach operating temperature. The O2 Sensor uses a Positive Thermal Coefficient (PTC) heater element. As temperature increases, resistance increases. At ambient temperatures around 21 °C (70 °F), the resistance of the heating element is approximately 6 ohms for the Downstream Sensor and is approximately 4.5 ohms for the Upstream Sensor. As the sensor's temperature increases, resistance in the heater element increases.
Upstream Oxygen Sensor 1/1
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. Separate controlled ground circuits are run through the PCM for the upstream O2 sensors.
The sensor input switches from 0 to 1 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 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 oxygen sensors are equipped with a heating element that keeps the sensors at proper operating temperature during all operating modes. 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 operation the PCM monitors the O2S 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.
Downstream Oxygen Sensor 1/2
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. If the oxygen leaving the catalyst is too lean (excess oxygen), the PCM increases the upstream O2 goal, which increases fuel in the mixture, causing less oxygen to be left over. Conversely, if the oxygen content leaving the catalyst is too rich (not enough oxygen) the PCM decreases the upstream O2 goal down which removes fuel from the mixture causing more oxygen to be left over. This function only occurs during downstream closed loop mode operation.