Wrangler 4WD L4-150 2.5L VIN P MFI (1998)
Oxygen Sensor: Description and Operation
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
