Task Manager Description And Operation; Test Sequence; Mil Illumination; Diagnostic Trouble Codes (Dtcs) - Jeep 200 Cherokee Service Manual

25 - 16 EMISSION CONTROL SYSTEMS
DESCRIPTION AND OPERATION (Continued)
TASK MANAGER
DESCRIPTION
The PCM is responsible for efficiently coordinating
the operation of all the emissions-related compo-
nents. The PCM is also responsible for determining if
the diagnostic systems are operating properly. The
software designed to carry out these responsibilities
is call the 'Task Manager'.
OPERATION
The Task Manager determines which tests happen
when and which functions occur when. Many of the
diagnostic steps required by OBD II must be per-
formed under specific operating conditions. The Task
Manager software organizes and prioritizes the diag-
nostic procedures. The job of the Task Manager is to
determine if conditions are appropriate for tests to be
run, monitor the parameters for a trip for each test,
and record the results of the test. Following are the
responsibilities of the Task Manager software:

• Test Sequence

• MIL Illumination

• Diagnostic Trouble Codes (DTCs)

• Trip Indicator
• Freeze Frame Data Storage
• Similar Conditions Window
Test Sequence
In many instances, emissions systems must fail
diagnostic tests more than once before the PCM illu-
minates the MIL. These tests are know as 'two trip
monitors.' Other tests that turn the MIL lamp on
after a single failure are known as 'one trip moni-
tors.' A trip is defined as 'start the vehicle and oper-
ate it to meet the criteria necessary to run the given
monitor.'
Many of the diagnostic tests must be performed
under certain operating conditions. However, there
are times when tests cannot be run because another
test is in progress (conflict), another test has failed
(pending) or the Task Manager has set a fault that
may cause a failure of the test (suspend).
• Pending
Under some situations the Task Manager will not
run a monitor if the MIL is illuminated and a fault is
stored from another monitor. In these situations, the
Task Manager postpones monitors pending resolu-
tion of the original fault. The Task Manager does not
run the test until the problem is remedied.
For example, when the MIL is illuminated for an
Oxygen Sensor fault, the Task Manager does not run
the Catalyst Monitor until the Oxygen Sensor fault is
remedied. Since the Catalyst Monitor is based on sig-
nals from the Oxygen Sensor, running the test would
produce inaccurate results.
• Conflict
There are situations when the Task Manager does
not run a test if another monitor is in progress. In
these situations, the effects of another monitor run-
ning could result in an erroneous failure. If this con-
flict is present, the monitor is not run until the
conflicting condition passes. Most likely the monitor
will run later after the conflicting monitor has
passed.
For example, if the Fuel System Monitor is in
progress, the Task Manager does not run the EGR
Monitor. Since both tests monitor changes in air/fuel
ratio and adaptive fuel compensation, the monitors
will conflict with each other.
• Suspend
Occasionally the Task Manager may not allow a two
trip fault to mature. The Task Manager will sus-
pend the maturing of a fault if a condition exists
that may induce an erroneous failure. This prevents
illuminating the MIL for the wrong fault and allows
more precis diagnosis.
For example, if the PCM is storing a one trip fault
for the Oxygen Sensor and the EGR monitor, the
Task Manager may still run the EGR Monitor but
will suspend the results until the Oxygen Sensor
Monitor either passes or fails. At that point the Task
Manager can determine if the EGR system is actu-
ally failing or if an Oxygen Sensor is failing.
MIL Illumination
The PCM Task Manager carries out the illumina-
tion of the MIL. The Task Manager triggers MIL illu-
mination upon test failure, depending on monitor
failure criteria.
The Task Manager Screen shows both a Requested
MIL state and an Actual MIL state. When the MIL is
illuminated upon completion of a test for a third trip,
the Requested MIL state changes to OFF. However,
the MIL remains illuminated until the next key
cycle. (On some vehicles, the MIL will actually turn
OFF during the third key cycle) During the key cycle
for the third good trip, the Requested MIL state is
OFF, while the Actual MILL state is ON. After the
next key cycle, the MIL is not illuminated and both
MIL states read OFF.
Diagnostic Trouble Codes (DTCs)
With OBD II, different DTC faults have different
priorities according to regulations. As a result, the
priorities determine MIL illumination and DTC era-
sure. DTCs are entered according to individual prior-
ity. DTCs with a higher priority overwrite lower
priority DTCs.
Priorities
• Priority 0 —Non-emissions related trouble codes
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