6E-16 EMISSION AND ELECTRICAL DIAGNOSIS
General Service Information
On Board Diagnostic (OBD) Serviceability Issues
The list of non-vehicle faults that could affect the perfor-
mance of the OBD system has been complied. These
non-vehicle faults vary from environmental conditions to
the quality of fuel used.
The illumination of the Malfunction Indicator Lamp (MIL)
(“Check Engine” lamp) due to a non-vehicle fault could
lead to misdiagnosis of the vehicle, increased warranty
expense and customer dissatisfaction. The following list
of non-vehicle faults does not include every possible
fault and may not apply equally to all product lines.
Poor Vehicle Maintenance
The sensitivity of OBD diagnostics will cause the MIL to
turn on if the vehicle is not maintained properly. Restrict-
ed air filters, fuel filters, oil filters, and crankcase depos-
its due to lack of oil changes or improper oil viscosity can
trigger actual vehicle faults that were not previously
monitored prior to OBD. Poor vehicle maintenance can
not be classified as a “non-vehicle fault”, but with the
sensitivity of OBD diagnostics, vehicle maintenance
schedules must be more closely followed.
Maintenance Schedule
Refer to the maintenance Schedule.
Visual/Physical Engine Compartment Inspection
Perform a careful visual and physical engine compart-
ment inspection when performing any diagnostic proce-
dure or diagnosing the cause of an emission test failure.
This can often lead to repairing a problem without further
steps. Use the following guidelines when performing a
visual/physical inspection:
• Inspection all vacuum hoses for punches, cuts, dis-
connects, and correct rouring.
• Inspect hoses that are difficult to see behind other
components.
• Inspect all wires in a engine compartment for prop-
er connections, burned or chafed spots, pinched
wires, contact with sharp edges or contact with hot
exhaust manifolds or pipes.
Basic Knowledge of Tools Required
Notice:
Lack of basic knowledge of this powertrain when per-
forming diagnostic procedures could result in an incor-
rect diagnosis or damage to powertrain components. Do
not attempt to diagnose a powertrain problem without
this basic knowledge.
A basic understanding of hand tools is necessary to ef-
fectively use this section of the Service Manual.
On-Board Diagnostic (OBD)
On-Board Diagnostic Tests
A diagnostic test is a series of steps, the result of which
is a pass or fail reported to the diagnostic executive.
When a diagnostic test reports a pass result, the diag-
nostic executive records the following data:
• The diagnostic test has been completed since the
last ignition cycle.
• The diagnostic test has passed during the current
ignition cycle.
• The fault identified by the diagnostic test is not cur-
rently active.
When a diagnostic test reports a fail result, the diagnos-
tic executive records the following data:
• The diagnostic test has been completed since the
last ignition cycle.
• The fault identified by the diagnostic test is current-
ly active.
• The fault has been active during this ignition cycle.
• The operating conditions at the time of the failure.
Common OBD Terms
Diagnostic
When used as a noun, the word diagnostic refers to any
on-board test run by the vehicle’s Diagnostic Manage-
ment System. A diagnostic is simply a test run on a sys-
tem or component to determine if the system or
component is operating according to specification.
Enable Criteria
The term “enable criteria” is engineering language for
the conditions necessary for a given diagnostic test to
run. Each diagnostic has a specific list of conditions
which must be met before the diagnostic will run. “En-
able criteria” is another way of saying “conditions re-
quired”.
Trip
Technically, a trip is a key on-run-key off cycle in which
all the enable criteria for a given diagnostic are met, al-
lowing the diagnostic to run. Unfortunately, this concept
is not quite that simple. A trip is official when all the en-
able criteria for a given diagnostic are met. But because
the enable criteria vary from one diagnostic to another,
the definition of trip varies as well. Some diagnostic are
run when the vehicle is at operating temperature, some
when the vehicle first start up; some require that the ve-
hicle be cruising at a steady highway speed, some run
only when the vehicle is idle.
Some run only immediately following a cold engine start-
up.
A trip then, is defined as a key on-run-key off cycle in
which the vehicle was operated in such a way as to sat-
isfy the enables criteria for a given diagnostic, and this
diagnostic will consider this cycle to be one trip. Howev-
er, another diagnostic with a different set of enable cri-
teria (which were not met) during this driving event,
would not consider it a trip. No trip will occur for that par-
ticular diagnostic until the vehicle is driven in such a way
as to meet all the enable criteria.