Chrysler 2005 Crossfire SRT6 Service Manual page 4298

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OPERATION - To monitor catalyst efficiency, the PCM expands the rich and lean switch points of the heated oxygen
sensor. With extended switch points, the air/fuel mixture runs richer and leaner to overburden the catalytic converter.
Once the test is started, the air/fuel mixture runs rich and lean and the O2S switches are counted. A switch is
counted when an oxygen sensor signal goes from below the lean threshold to above the rich threshold. The number
of Rear O2S switches is divided by the number of Front O2S switches to determine the switching ratio.
The test runs for 20 seconds. As catalyst efficiency deteriorates over the life of the vehicle, the switch rate at the
downstream sensor approaches that of the upstream sensor. If at any point during the test period the switch ratio
reaches a predetermined value, a counter is incremented by one. The monitor is enabled to run another test during
that trip. When the test fails three times, the counter increments to three, a malfunction is entered, and a Freeze
Frame is stored. When the counter increments to three during the next trip, the code is matured and the MIL is
illuminated. If the test passes the first, no further testing is conducted during that trip.
The MIL is extinguished after three consecutive good trips.
DESCRIPTION - MONITORED SYSTEMS
There are new electronic circuit monitors that check fuel, emission, engine and ignition performance. These moni-
tors use information from various sensor circuits to indicate the overall operation of the fuel, engine, ignition and
emission systems and thus the emissions performance of the vehicle.
The fuel, engine, ignition and emission system monitors do not indicate a specific component problem. They do
indicate that there is an implied problem within one of the systems and that a specific problem must be diagnosed.
If any of these monitors detect a problem affecting vehicle emissions, the Malfunction Indicator Lamp (MIL) will be
illuminated. These monitors generate Diagnostic Trouble Codes that can be displayed with the a DRB III scan tool.
The following is a list of the system monitors:
• EGR Monitor
• Misfire Monitor
• Fuel System Monitor
• Oxygen Sensor Monitor
• Oxygen Sensor Heater Monitor
• Catalyst Monitor
• Evaporative Vacuum Leak Detection System Monitor
Following is a description of each system monitor and its DTC.
Refer to the appropriate Powertrain Diagnostic Procedures manual for diagnostic procedures.
OXYGEN SENSOR (O2S) MONITOR
DESCRIPTION - Effective control of exhaust emissions is achieved by an oxygen feedback system. The most
important element of the feedback system is the O2S. The O2S is located in the exhaust path. Once it reaches
operating temperature 300° to 350°C (572° to 662°F), the sensor generates a voltage that is inversely proportional
to the amount of oxygen in the exhaust. When there is a large amount of oxygen in the exhaust caused by a lean
condition, misfire or exhaust leak, the sensor produces a low voltage, below 450mV. When the oxygen content is
lower, caused by a rich condition, the sensor produces a higher voltage, above 450mV.
The information obtained by the sensor is used to calculate the fuel injector pulse width. The PCM is programmed
to maintain the optimum air/fuel ratio. At this mixture ratio, the catalyst works best to remove hydrocarbons (HC),
carbon monoxide (CO) and nitrous oxide (NOx) from the exhaust.
The O2S is also the main sensing element for the EGR, Purge System, and Catalyst and Fuel Monitors.
The O2S may fail in any or all of the following manners:
• Slow response rate (Big Slope)
• Reduced output voltage (Half Cycle)
• Heater Performance
• Dynamic shift
• Shorted or open circuits
Slow Response Rate (Big Slope) - Response rate is the time required for the sensor to switch from lean to rich
signal output once it is exposed to a richer than optimum air/fuel mixture or vice versa. As the PCM adjusts the
air/fuel ratio, the sensor must be able to rapidly detect the change. As the sensor ages, it could take longer to detect
EMISSIONS CONTROL 25 - 5