Oxygen Sensor: Description and Operation
Fig 11 Upstream Heated Oxygen Sensor:
OPERATION
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 converter, one downstream of the converter. 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 automatic shutdown 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.
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 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 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.
The upstream oxygen sensor threads into the outlet flange of the exhaust manifold (Fig 11).
Fig 13 Downstream Heated Oxygen Sensor 1/2:
DOWNSTREAM OXYGEN SENSOR
The downstream heated oxygen sensor threads into the outlet pipe at the rear of the catalytic convertor (Fig. 13). 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.
CIRCUIT OPERATION
Circuit A14 is connected to the BUS bar in the Power Distribution Center (PDC), which connects to battery voltage. The contact side of the Automatic Shut-Down (ASD) relay connects circuit A14 and circuit A142. A 20 amp fuse, located in cavity 5 of the PDC, protects circuits A14 and A142.
Circuit A14 also supplies voltage to the coil side of the ASD relay. The Powertrain Control Module (PCM) controls the ground path circuit for the coil side of the ASD relay on circuit K51. Circuit K51 connects to cavity 67 of the PCM.
The A142 circuit also splices to cavity 6 of the PCM connector. The input by circuit A142 tells the PCM that the ASD relay is energized.
The A142 circuit supplies voltage for the upstream heated oxygen sensor and circuit F142 supplies voltage to the downstream heated oxygen sensor. The F142 circuit is protected by a 10 amp fuse located in cavity 1 of the PDC. Power for the fuse is supplied by circuit A142.
Circuit K41 delivers the signal from the upstream heated oxygen sensor to the PCM. Circuit K41 connects to cavity 30 of the PCM connector.
Circuit K141 delivers the signal from the downstream heated oxygen sensor to the PCM. Circuit K141 connects to cavity 51 of the PCM connector.
The PCM provides a ground for the upstream (left) and downstream (right) heated oxygen sensor signals (circuits K41 and K141) through circuit K4. Circuit K4 connects to cavity 43 of the PCM connector.
Circuit Z1 provides the ground path for the heater circuits in the sensors. This ground terminates at the right frame rail.
HELPFUL INFORMATION
Circuit A142 splices to supply voltage to the fuel injectors, ignition coil, ASD relay, generator, and the EGR solenoid. The PCM controls the ground path for each of the components powered by circuit A142.
Circuit K4 splices to supply ground for the signals for the following:
- Intake air temperature sensor
- Engine coolant temperature sensor
- Manifold absolute pressure sensor
- Throttle position sensor
- A/C pressure switch
- Camshaft position sensor
- Crankshaft position sensor
- Vehicle speed sensor