RAM 1500 Truck 2WD V6-3.9L VIN X (1999)
Maintaining correct sensor temperature at all times 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 sensor 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.
The Automatic Shutdown (ASD) relay supplies battery voltage to both oxygen sensors. The oxygen sensors are equipped with a heating elements.
The heating elements reduce the time required for the sensors to reach operating temperature.
PRE-CATALYST OXYGEN SENSOR
Pre-Catalyst/post Catalyst Oxygen Sensors
The pre-catalyst O2S sensor is located in the inlet end of the catalytic converter. It provides an input voltage to the PCM. By comparing the input
from the pre-catalyst O2S sensor, with the input from the post catalyst oxygen sensor, the PCM calculates catalytic convertor efficiency.
POST CATALYST OXYGEN SENSOR
The post catalyst heated oxygen sensor threads into the outlet end of the catalytic convertor. The post catalyst heated oxygen sensor input is used
to detect catalytic convertor deterioration and fine tune the air fuel ratio. As the convertor deteriorates, the input from this sensor begins to match
the precatalyst sensor input except for a slight time delay. By comparing the inputs from both of these sensors, 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.
