Hummer H1 Service Manual page 1079

14 PCM/Tech 1 Scan Tool
DTC (DTC) IDENTIFICATION
The MIL (Service Engine Soon) lamp will be "ON" if an emis-
sion malfunction exists. If the malfunction clears, the lamp will
go "OFF" and the DTC will be stored in the PCM. Any DTCs
stored will be cleared if no problem recurs within l50 engine
starts.
• All DTCs with the sign * are transmission related DTCs
and have descriptions, diagnostic charts are in Section 5.
Remember, always start with the lowest numerical en-
gine DTC first. When diagnosing some engine DTCs,
other transmission symptoms can occur.
Intermittents
A corresponding DTC will be stored in the memory of the
PCM as a history DTC until DTCs have been cleared. When
unexpected DTCs appear during the code reading process, one
can assume that these DTCs were set by an intermittent mal-
function and could be helpful in diagnosing the system. An in-
termittent DTC may or may not re-set. If it is an intermittent
failure, a Diagnostic Trouble Code (DTC) chart is not used.
Consult the "Diagnostic Aids" on the page facing the diagnos-
tic chart corresponding to the intermittent DTC. Section "2"
also covers the topic of "Intermittents". A physical inspection
of the applicable sub-system most often will not resolve the
problem.
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Scan Tool Use With Intermittents
In some scan tool allocations, the data update rate makes the
tool less effective than a voltmeter, such as when trying to de-
tect an intermittent problem which lasts for a very short time.
However, the scan tool allows manipulation of wiring har-
nesses or components under the hood with the engine not run-
ning, while observing the scan tool readout.
The scan tool can be plugged in and observed while driving the
vehicle under the condition when the MIL "Service Engine
Soon" light turns "ON" momentarily or when the engine drive-
ability is momentarily poor. If the problem seems to be related
to certain parameters that can be checked on the scan tool, they
should be checked while driving the vehicle. If there does not
seem to be any correlation between the problem and any spe-
cific circuit, the scan tool can be checked on each position,
watching for a period of time to see if there is any change in the
reading that indicates intermittent operation.
The scan tool is also an easy way to compare the operating pa-
rameters of a poorly operating engine with those of a known
good one. For example, a sensor may shift in value but not set a
DTC. Comparing the sensor's readings with those of a known
good vehicle may uncover the problem.
The scan tool has the ability to save time in diagnosis and pre-
vent the replacement of good parts. The key to using the scan
tool successfully for diagnosis lies in the technician's ability to
understand the system he is trying to diagnose as well as under-
standing of the scan tool operation and limitations. The techni-
cian should read the tool manufacturer's operating manual to
become familiar with the tool's operation.
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