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).
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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 resolution 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 signals
from the Oxygen Sensor, running the test would produce inaccurate results.
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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 running could result in an erroneous failure. If this conflict is present,
the monitor is not run until the conflicting condition passes. Most likely the monitor will run later after the con-
flicting 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.
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Suspend
Occasionally the Task Manager may not allow a two trip fault to mature. The Task Manager will suspend 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 actually failing or if an
Oxygen Sensor is failing.
MIL Illumination
The PCM Task Manager carries out the illumination of the MIL. The Task Manager triggers MIL illumination 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 deter-
mine MIL illumination and DTC erasure. DTCs are entered according to individual priority. DTCs with a higher pri-
ority overwrite lower priority DTCs.
Priorities
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Priority 0 —Non-emissions related trouble codes
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Priority 1 — One trip failure of a two trip fault for non-fuel system and non-misfire.
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Priority 2 — One trip failure of a two trip fault for fuel system (rich/lean) or misfire.
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Priority 3 — Two trip failure for a non-fuel system and non-misfire or matured one trip comprehensive com-
ponent fault.
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Priority 4 — Two trip failure or matured fault for fuel system (rich/lean) and misfire or one trip catalyst dam-
aging misfire.
Non-emissions related failures have no priority. One trip failures of two trip faults have low priority. Two trip failures
or matured faults have higher priority. One and two trip failures of fuel system and misfire monitor take precedence
over non-fuel system and non-misfire failures.
DTC Self Erasure
With one trip components or systems, the MIL is illuminated upon test failure and DTCs are stored.
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EMISSIONS CONTROL
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