Heated Oxygen Sensors

Fig 18 Heated Oxygen Sensor:

GENERAL OPERATION
The 02 sensors produce voltages 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 sensors produces a low voltage.
- When there is a lesser amount present (rich air/tiel mixture) it produces a higher voltage.

By monitoring the oxygen content and converting it to electrical voltage, the sensors act as a rich-lean switch.

Oxygen Sensor Heaters
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. This also allows the system to remain in closed loop operation during periods of extended idle.

The Automatic Shutdown (ASD) relay supplies battery voltage to both the upstream and downstream oxygen sensor heaters.

Closed Loop Operation
In Closed Loop operation the PCM monitors the 025 sensor input (along with other inputs) and adjusts the injector pulse width accordingly.

Open Loop Operation
During Open Loop operation the PCM ignores the 02 sensor input. The PCM adjusts injector pulse width based on preprogrammed (fixed) values and inputs from other sensors.

UPSTREAM HEATED OXYGEN SENSOR
The upstream 025 sensor is located in the exhaust manifold and provides an input voltage to the PCM. The input tells the PCM the oxygen content of the exhaust gas (Fig. 18). The PCM uses this information to fine tune the air/fuel ratio by adjusting injector pulse width.

Fig 19 Upstream Heated Oxygen Sensor:

DOWNSTREAM HEATED OXYGEN SENSOR
The downstream heated oxygen sensor threads into the out]et pipe at the rear of the catalytic convertor (Fig. 19). 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.

When the catalytic converter efficiency drops below emission standards, the PCM stores a diagnostic trouble code and illuminates the Malfunction Indicator Lamp (MIL). For more information, refer to Emission Control Systems.