Oxygen Sensor (O2S)-PCM Input

OXYGEN SENSOR (O2S)-PCM INPUT

Two heated Oxygen Sensors (O2S) are used. When the key is turned ON, and the engine is cold, the sensors will have an initial voltage of 5 volts. The sensors themselves 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/fuel 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.

Both oxygen sensors are equipped with a heating element that reduces the time required for the sensors to reach operating temperature allowing the system to enter into closed loop operation sooner. Maintaining correct sensor temperature at all times 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 heating elements.

In Closed Loop operation, the Powertrain Control Module (PCM) monitors both O25 sensor inputs (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.

UPSTREAM HEATED OXYGEN SENSOR

Fig. 5 Upstream Oxygen Sensor:

The upstream O2S sensor is located in the exhaust downpipe (Fig. 5). It provides 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 the air/fuel ratio by adjusting injector pulse width.

DOWNSTREAM HEATED OXYGEN SENSOR

Fig. 6 Downstream Oxygen Sensor:

The downstream heated oxygen sensor is located near the outlet end of the catalytic convertor (Fig. 6). The downstream heated oxygen sensor input is used to detect catalytic convertor deterioration and provide fuel adjustment information.

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 (MIL) (or Check Engine) lamp. For more information, refer Emission Control Systems.

The downstream sensor input is also used to adjust the upstream O2S goal voltage. This allows a better control of the exhaust gases as the catalytic convertor ages.

NOTE: The upstream and downstream sensors may look exactly alike, but are not interchangeable.