Medium Duty Engines

Fig. 11 Pre-Catalyst/Post Catalyst Oxygen Sensors - MDC Engines:

DESCRIPTION
The vehicle, when equipped with a Medium Duty Emission Cycle (MDC) package, will use four heated Oxygen (O2S) sensors. They are: Left, right, pre-catalyst and post catalyst. The left, right and post catalyst sensors will fine-tune air-fuel ratio. The pre-catalyst and post catalyst sensors will determine catalytic converter efficiency.

LOCATION
Two of these sensors are installed into the left and right exhaust manifold downpipes.

OPERATION
The left O2S sensor will monitor cylinders 1, 3, 5, 7 and 9. The right sensor will monitor cylinders 2, 4, 6, 8 and 10. The 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/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.

HEATING ELEMENT
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. Also, it allows the system to remain in closed loop operation during periods of extended idle.

OPERATING MODES
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 02 sensor input. The PCM adjusts injector pulse width based on preprogrammed (fixed) values and inputs from other sensors.

ASD RELAY
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
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).