Hummer H1 Service Manual page 1067

2 PCM/Tech 1 Scan Tool
Diagnostic Information
The diagnostic table and functional checks in this manual are
designed to locate a faulty circuit or component through logic
based on the process of elimination.
The charts are prepared with the requirements that the vehicle
functioned correctly at the time of assembly and that there are
no multiple failures.
The PCM performs a continual self-diagnosis on certain con-
trol functions. This diagnostic capability is complemented by
the diagnostic procedures contained in this manual. The
PCM's language for communicating the source of a malfunc-
tions is a system of DTCs.
Malfunction Indicator Lamp (MIL) "Check
Engine"
This light is on the instrument panel and has the following
functions.
• It informs the driver that a problem has occurred and
that the vehicle should be taken in for service as soon as
reasonably possible.
• It displays DTCs stored by the PCM which help the
technician diagnose system problems.
As a bulb and system check, the light will come "ON" with the
key "ON" and the engine not running. When the engine is
started, the light will turn "OFF." If the light remains "ON,"
the self-diagnostic system has detected a problem. If the prob-
lem goes away, the light will go out in most cases after 10 sec-
onds, but a DTC will remain stored in the PCM.
When the light remains "ON" while the engine is running, or
when a malfunction is suspected due to a driveability or emis-
sions problem, an "On-Board Diagnostic (OBD) System
Check" must be performed. This check will expose malfunc-
tions which may not be detected if other diagnostics are per-
formed prematurely.
"Check Throttle" Lamp
This light is on the instrument panel and has the following
functions:
• It informs the driver that a problem has occurred in the
Accelerator Pedal Position (APP) circuit and the vehicle
should be taken in for service as soon as reasonably pos-
sible.
• If APP DTCs are stored by the PCM, the MIL will dis-
play these. The "Check Throttle" Lamp will not display
DTCs.
As a bulb and system check, the light will come "ON" with the
key "ON" for 2 seconds. When the engine is started, the light
will turn "OFF." If the light remains "ON," the self-diagnostic
system has detected a problem. If the problem goes away, the
light will go out in most cases after 10 seconds, but a DTC will
remain stored in the PCM.
When the light remains "ON" while the engine is running, or
when a malfunction is suspected, an "On-Board Diagnostic
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(OBD) System Check" must be performed. This check will ex-
pose malfunctions which may not be detected if other diagnos-
tics are performed prematurely.
Circuit Description
The "On-Board diagnostic (OBD) System Check" is an orga-
nized approach to identifying a problem created by a control
module system malfunction. It must be the starting point for
any driveability complaint diagnosis, this will direct the ser-
vice technician to the next logical step in diagnosing the com-
plaint. Understanding the chart and using it properly will
reduce diagnostic time and prevent the unnecessary replace-
ment of good parts.
Test Description
Number(s) below refer to the step number(s) on the Diagnostic
Table.
1. When the ignition switch is cycled to "ON," the MIL should
2. Use Scan Tool to aid diagnosis, therefore, serial data must
be available.
3. Although the PCM is powered up, a "Cranks But Will Not
Run" symptom could exist because of a PCM or system
problem.
4. Comparison of actual control system data with the Typical
Scan Tool Data Values is a quick check to determine if any
parameter is not within limits. A base engine problem (i.e.,
advanced cam timing) may substantially alter sensor values.
5. This step will isolate if the customer complaint is a MIL or
driveability problem with no MIL. Refer to "DTC Identifi-
cation" in this Section for a list of valid DTCs. An invalid
DTC may be the result of a faulty Scan Tool, PROM or
PCM.
turn "ON" and remain "ON" steady. This sequence will
determine that the vehicle diagnostics are operational.
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