ENGINE <NON-TURBO>
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Control System
1-33
DTC Identification, Maturation, and Erasure
ii
Once a test has been run, the diagnostic system
determines whether the system has passed or failed.
It must then determine if the test has failed the
specified number of times required to illuminate the
MIL. If not, the diagnostic system stores a maturing
code. When this test is run again (on the next “trip”)
the results are once again either pass or fail. If
the component fails the test, a code “matures,” and
a DTC is set. The MIL lamp is illuminated if an
emission component is involved. If the component
or system passes the test a specified number of
times, the maturing code is erased.
NOTE
It is important to understand that a system need
not fail a test under exactly the same conditions
for a code to nature and trigger the MIL.
Freeze Frame Data Storage and Erasure
All monitored systems provide “freeze frame” data
of the vehicle’s operating conditions when a matur-
ing code was set. This information can be retrieved
using the diagnostic scan tool and may help a techni-
cal pinpoint the source of a concern more quickly.
When there are multiple monitor failures, the first
to occur is stored in freeze frame. The exceptions
L
are the misfire and fuel system monitors. They have
priority and can write over other freeze frame data.
Freeze Frame Priority
The freeze frame only indicates the operating condi-
tions under which the maturing code was set. While
these conditions are usually the same as those
which trigger the maturation of the DTC, there can
be exceptions. This first condition is important for
fuel system and misfire DTC’s as it is the condition
MIL Illumination
As described earlier, the PCM diagnostic system
is responsible for operation of the MIL lamp. This
is an important function as unnecessary operation
of this lamp has a negative effect on customer satis-
faction. In most cases, if a component or system
passes diagnostics on three consecutive trips after
the MIL has been illuminated, the MIL is extin-
guished. This explains how customers can note
that the “Check Engine” light went on, and then
off, on its own. The OBD II has reconsidered its
diagnosis. The DTC remains in the memory until
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DTC’s are erased if the component passes the diag-
nostic test in the course of a number of subsequent
warm-up cycles. The number of trips required for
this is usually 40. This is why it is best to attempt
to diagnose intermittent problems soon after they
occur. While initially a DTC may have been available,
subsequent trips (which passed the diagnostic test)
have erased the code. The OBD II system, based
on the latest inputs it received, reconsidered its
diagnosis on this intermittent concern.
Of course, a diagnostic scan tool can be used to
erase DTC’s at any time. In addition, DTC’s (along
with records of successful trips) are erased whenev-
er the vehicle’s battery is disconnected.
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Information in the freeze frame includes:
l
Diagnostic Trouble Code (DTC)
l Engine rpm
l Engine load
l
Fuel trim (short term and long term)
o
Engine coolant temperature
l
Manifold absolute pressure
l Operation mode (open or closed loop)
l Vehicle speed
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the vehicle must repeat successfully during a test
to allow the diagnostic system to erase a DTC and
turn off the MIL on its own.
Should the diagnostic system determine that the
DTC can be erased, the freeze frame related to
that code is also eliminated.
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AK37AA
a larger number of warm-up cycles (40) have been
successfully completed.
The MIL flashed in misfire situations where contin-
ued driving could damage the catalytic converter.
Continued driving of the vehicle when this condition
occurs is not recommended.
The MIL can be used to produce diagnostic trouble
codes. Quickly turning the key to the RUN position
three times allows the MIL to flash out any existing
flash codes.
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