must stay below that threshold for a calibrated
amount of time.
• Test Done This Trip — Indicates that the
monitor has already been run and completed during
the current trip.
MISFIRE
• Same Misfire Warm-Up State — Indicates if
the misfire occurred when the engine was warmed up
(above 160° F).
• In Similar Misfire Window — An indicator
that ’Absolute MAP When Misfire Occurred’ and
’RPM When Misfire Occurred’ are all in the same
range when the failure occurred. Indicated by switch-
ing from ’NO’ to ’YES’.
• Absolute MAP When Misfire Occurred —
The stored MAP reading at the time of failure.
Informs the user at what engine load the failure
occurred.
• Absolute MAP — A live reading of engine load
to aid the user in accessing the Similar Conditions
Window.
• RPM When Misfire Occurred — The stored
RPM reading at the time of failure. Informs the user
at what engine RPM the failure occurred.
• Engine RPM — A live reading of engine RPM
to aid the user in accessing the Similar Conditions
Window.
• Adaptive Memory Factor — The PCM uti-
lizes both Short Term Compensation and Long Term
Adaptive to calculate the Adaptive Memory Factor
for total fuel correction.
• 200 Rev Counter — Counts 0–100 720 degree
cycles.
• SCW Cat 200 Rev Counter — Counts when in
similar conditions.
• SCW FTP 1000 Rev Counter — Counts 0–4
when in similar conditions.
• Misfire Good Trip Counter — Counts up to
three to turn OFF the MIL.
• Misfire Data — Data collected during test.
• Test Done This Trip — Indicates YES when
the test is done.
MONITORED SYSTEMS
OPERATION
There are new electronic circuit monitors that
check fuel, emission, engine and ignition perfor-
mance. These monitors 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 systems
monitors do not indicate a specific component prob-
lem. They do indicate that there is an implied prob-
lem 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 MIL or a scan tool.
The following is a list of the system monitors:
• Misfire Monitor
• Fuel System Monitor
• Oxygen Sensor Monitor
• Oxygen Sensor Heater Monitor
• Catalyst Monitor
• Leak Detection Pump Monitor (if equipped)
All these system monitors require two consecutive
trips with the malfunction present to set a fault.
Refer to the appropriate Powertrain Diagnos-
tics Procedures manual for diagnostic proce-
dures.
The following is an operation and description of
each system monitor :
OXYGEN SENSOR (O2S) MONITOR
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 oper-
ating temperature 300° to 350°C (572° to 662°F), the
sensor generates a voltage that is inversely propor-
tional to the amount of oxygen in the exhaust. The
information obtained by the sensor is used to calcu-
late the fuel injector pulse width. This maintains a
14.7 to 1 Air Fuel (A/F) ratio. At this mixture ratio,
the catalyst works best to remove hydrocarbons (HC),
carbon monoxide (CO) and nitrogen oxide (NOx) from
the exhaust.
The O2S is also the main sensing element for the
Catalyst and Fuel Monitors.
The O2S can fail in any or all of the following
manners:
• slow response rate
• reduced output voltage
• dynamic shift
• shorted or open circuits
Response rate is the time required for the sensor to
switch from lean to rich once it is exposed to a richer
than optimum A/F mixture or vice versa. As the sen-
sor starts malfunctioning, it could take longer to
detect the changes in the oxygen content of the
exhaust gas.
The output voltage of the O2S ranges from 0 to 1
volt. A good sensor can easily generate any output
voltage in this range as it is exposed to different con-
centrations of oxygen. To detect a shift in the A/F
mixture (lean or rich), the output voltage has to
change beyond a threshold value. A malfunctioning
DN EMISSION CONTROL SYSTEMS 25 - 19
DESCRIPTION AND OPERATION (Continued)
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