Chrysler 2005 Crossfire SRT6 Service Manual page 4297

25 - 4 EMISSIONS CONTROL
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The Oxygen Sensor Signal Monitor is a 2 trip monitor that is tested only once per trip. When the Oxygen Sensor
fails the test in two consecutive trips, the MIL is illuminated and a DTC is set. The MIL is extinguished when the
Oxygen Sensor monitor passes in three consecutive trips. The DTC is erased from memory after 40 consecutive
warm-up cycles without test failure.
OXYGEN SENSOR HEATER MONITOR
DESCRIPTION - If the Oxygen Sensor (O2S) DTC as well as a O2S heater DTC is present, the O2S Heater DTC
MUST be repaired first. After the O2S Heater is repaired, verify that the sensor circuit is operating correctly.
Note: The O2S Heaters are kept off at coolant temperatures below 20°C (68°F) and at high engine rpm in
order to avoid damaging the heaters. The voltage reading taken from the O2S are very temperature sensitive.
The readings taken from the O2S are not accurate below 300°C (572°F). Heating the O2S is done to allow the
engine controller to shift to closed loop control as soon as possible. The heating element used to heat the O2S
must be tested to ensure that it is heating the sensor properly. The heater resistance is checked by the PCM almost
immediately after the engine is started. The same O2S heater return pin used to read the heater resistance is capa-
ble of detecting an open, shorted high or shorted low circuit.
OPERATION - The Oxygen Sensor Heater Monitor begins after the ignition has been turned OFF and the O2 sen-
sors have cooled. As the sensor cools down, the resistance increases and the PCM reads the increase in voltage.
Once voltage has increased to a predetermined amount, higher than when the test started, the oxygen sensor is
cool enough to test heater operation.
When the oxygen sensor is cool enough, the PCM provides a ground path for the O2S heater circuit. Voltage to the
O2 sensor begins to increase the temperature. As the sensor temperature increases, the internal resistance
decreases.
The heater elements are tested each time the engine is turned OFF if all the enabling conditions are met. If the
monitor fails, the PCM stores a maturing fault and a Freeze Frame is entered. If two consecutive tests fail, a DTC
is stored. Because the ignition is OFF, the MIL is illuminated at the beginning of the next key cycle, after the 2nd
failure.
CATALYST MONITOR
DESCRIPTION - To comply with clean air regulations, vehicles are equipped with catalytic converters. These con-
verters reduce the emission of hydrocarbons, oxides of nitrogen and carbon monoxide.
Normal vehicle miles or engine misfire can cause a catalyst to decay. A meltdown of the ceramic core can cause a
restriction of the exhaust. This can increase vehicle emissions and deteriorate engine performance, driveability and
fuel economy.
The catalyst monitor uses dual oxygen sensors (O2Ss) to monitor the efficiency of the converter. The dual O2S
strategy is based on the fact that as a catalyst deteriorates, its oxygen storage capacity and its efficiency are both
reduced. By monitoring the oxygen storage capacity of a catalyst, its efficiency can be indirectly calculated. The
upstream O2S is used to detect the amount of oxygen in the exhaust gas before the gas enters the catalytic con-
verter. The PCM calculates the air/fuel mixture from the output of the O2S. A low voltage indicates high oxygen
content (lean mixture). A high voltage indicates a low content of oxygen (rich mixture).
When the upstream O2S detects a high oxygen condition, there is an abundance of oxygen in the exhaust gas. A
functioning converter would store this oxygen so it can use it for the oxidation of HC and CO. As the converter
absorbs the oxygen, there will be a lack of oxygen downstream of the converter. The output of the downstream O2S
will indicate limited activity in this condition.
As the converter loses the ability to store oxygen, the condition can be detected from the behavior of the down-
stream O2S. When the efficiency drops, no chemical reaction takes place. This means the concentration of oxygen
will be the same downstream as upstream. The output voltage of the downstream O2S copies the voltage of the
upstream sensor. The only difference is a time lag (seen by the PCM) between the switching of the O2Ss.
To monitor the system, the number of lean-to-rich switches of upstream and downstream O2Ss is counted. The ratio
of downstream switches to upstream switches is used to determine whether the catalyst is operating properly. An
effective catalyst will have fewer downstream switches than it has upstream switches i.e., a ratio closer to zero. For
a totally ineffective catalyst, this ratio will be one-to-one, indicating that no oxidation occurs in the device.
The system must be monitored so that when catalyst efficiency deteriorates and exhaust emissions increase to over
the legal limit, the MIL will be illuminated.