Chapter
E
: Computerized Engine Control Diagnosis And Repair (Including OBD II)
86
to the appropriate Service or Diagnostic Manual for
complete test procedures.
If the MIL lights steadily with the engine running, it indi-
cates that a system problem exists and a DTC has been
recorded. Not all system faults light the MIL. With some
faults, the MIL may flash briefly when the code is record-
ed, then go out.
A flashing MIL while driving an OBD II vehicle indicates
a serious misfire condition which will damage the cat-
alytic converter. Vehicles exhibiting this symptom
should not be driven. Converter overheating may occur.
Other problems may set a trouble code in memory but
not light the MIL at all. Code setting conditions vary by
manufacturer, year, and model. Accurate service infor-
mation for the vehicle being tested is required for trou-
bleshooting.
Diagnostic Trouble Codes
DTCs identify faults in ECM system sensors and circuits
or indicate individual system conditions. An OBD II DTC
is a five-character, alphanumeric fault identifier, figure
5-3. Since a letter is included in every DTC, the only way
to retrieve codes is with a scan tool.
The first character of an OBD II DTC is a letter. Refer to
figure 5-3 for an explanation of other letter codes.
The second character is a number that indicates if the
code is common to all OBD II vehicles (0) or specific to
one vehicle manufacturer (1). Remember, only emis-
sions related, P0 codes will activate the MIL.
The third character is a number used by all manufactur-
ers to identify which system has a fault. This designa-
tion will be the same for P0 (OBD II) or P1 (manufactur-
er's) codes. Following is the established numbering
system:
• 1 - Air/Fuel metering system input faults
• 2 - Air/Fuel metering output faults
• 3 - Ignition system or misfire faults
• 4 - Auxiliary emission controls
• 5 - Vehicle speed control and idle control system
• 6 - Computer output circuit faults
• 7 - Transmission
• 8 - Transmission
The fourth and fifth characters indicate the actual
problem associated with the code, (e.g. signal voltage
low, system always lean, etc.)
The intent of OBD II code designation is to help the
technician identify the system at fault, then pinpoint the
actual problem or specific circuit causing the fault.
Once a problem is identified by code, the technician
must use appropriate service manuals to complete the
diagnosis and repair.
Diagnostic Strategy
The most valuable aspect of diagnosis with a scan tool
is the ability to compare data from many sensors and
actuators. However, scan tool data should not be used
alone. Vehicle symptoms, driving conditions, and an
understanding of operational principles are also
important diagnostic tools.
Today's vehicles require today's technicians to be
aware of the ways traditional technology blends with
newer, more complex system-based technologies.
Vehicles manufactured before the 1970s controlled fuel
and ignition timing through vacuum and mechanical
weights and springs. Exhaust emissions were not
seriously considered until the early 1970s. Modern
vehicles use computerized controls to control fuel and
ignition timing precisely. The tradeoff, however, for this
technological advancement, is that today's drivetrain
problems can result in repeated and multiple
components failures that require a system-based
approach. For example, a late model vehicle has the
following symptoms: hard starting when cold, an
engine miss, and a failed emission test. The initial
diagnosis finds a fouled spark plug. Replacing the
spark plug and retesting emissions results in a passing
report and a smoothly running engine. While this
approach addresses the immediate symptom, it does
not deal with the underlying cause of the fouled spark
plug.
Fig. 5-3.
OBD II DTC code components.
Summary of Contents for ASE-A8
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