What is OBD and
what are its benefits?
OBD stands for "on-board diagnos-
tics," a computer-based system built
into all model year (MY) 1996 and
newer light-duty cars and trucks.
OBD monitors the performance of
some of the engines' major compo-
nents, including individual emis-
sion controls. The system provides
owners with an early warning of
malfunctions by way of a dashboard
"Check Engine" light (also known
as a Malfunction Indicator Light or
MIL, for short). By giving vehicle
owners this early warning, OBD
protects not only the environment
but also consumers, identifying
minor problems before they
become major repair bills.
For example, by identifying a rela-
tively inexpensive repair like the
replacement of a malfunctioning
oxygen sensor, OBD can save the
owner the cost of replacing the cat-
alytic converter later. By helping to
ensure that the vehicle operates
within its original design specifica-
tions, OBD can help save con-
sumers money by making sure gaso-
line isn't wasted as a result of, for
example, a loose gas cap or incom-
plete combustion. OBD stores infor-
mation about the malfunction
detected and actually helps the
repair technician to diagnose and
fix the vehicle. When OBD checks
are performed as part of a state's
Inspection and Maintenance (I/M)
program, they can help save con-
sumers time, taking on average five
minutes or less in centralized pro-
grams.
Why are states
required to include
hecks as
part of their
inspection and
maintenance (I/M)
program(s)?
This computer-based early warning
system was required by the 1990
Clean Air Act (CAA) and comes
standard on all MY1996 and newer
light-duty cars and trucks. The
CAA also required that checks of
the on-board diagnostic (OBD) sys-
tem be included in all mandatory
I/M programs to help ensure that
vehicle owners take this early
warning seriously. Many states have
already incorporated OBD checks
into their I/M programs, and many
more are on schedule to do so
within the next few years.
Despite numerous improvements in
automotive technology, motor
vehicles continue to be a major
source of air pollution, accounting
for approximately 77 percent of
the carbon monoxide (CO) and
45 percent of the ozone-causing
nitrogen oxides (NOx) in our
nation's air. I/M programs help
reduce excess emissions by identify-
ing vehicles in need of repair and
requiring that they be fixed. For
MY1995 and older vehicles, the
most effective way to identify need-
ed repairs is by measuring tailpipe
emissions. In some cases, however,
a vehicle may need repair before
emissions increase. For MY1996
and newer vehicles, the OBD
system makes it possible to detect
malfunctions before it leads to
high emissions. The OBD comput-
er monitors a wide range of
emissions controls and lights the
"Check Engine" light if a problem
is detected.
Why can a vehicle
pass a tailpipe test
but fail an OBD test?
OBD and tailpipe testing are two
different approaches to identify
vehicles in need of repair. The
OBD system looks for broken or
malfunctioning emissions control
components while tailpipe tests
sample a vehicle's exhaust to see if
it is above or below certain pre-
scribed limits. Given the robust
nature of today's emissions control
components, it is entirely possible
for an individual component to
malfunction without leading to an
immediate increase in emissions at
the tailpipe. In such cases, other
components (like the catalyst) can
Web site: www.epa.gov/otaq/obd.htm
E-mail: obd@epa.gov
Office of Transportation and Air Quality
EPA420-F-02-014
^ Printed on paper that contains at least 50 percent postconsumer fiber.
August 2002
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temporarily compensate for the
part that is broken; however, these
other components can only do dou-
ble duty for so long before they,
too, begin to malfunction. Because
of its ability to monitor individual
components, OBD is able to give
motorists an "early warning" that
repairs are needed; it is because of
this "early warning" capability that
OBD will sometimes fail vehicles
that would otherwise pass a tailpipe
test. In addition, OBD also moni-
tors for leaks and other malfunc-
tions in the fuel system—problems
that traditional tailpipe tests were
not designed to identify. Most state
and local areas also include a gas
cap pressure test as part of an emis-
sion inspection.
Are repair costs
going to increase?
Thus far, no increase has been
seen. Real world experience from
operating programs has shown
that the average cost for an OBD-
triggered repair is comparable to
that associated with repairs trig-
gered as a result of more traditional
tailpipe testing of OBD-equipped
vehicles. Regardless of the test used
to identify a failure in the I/M lane,
it is standard operating procedure
for a repair technician to consult a
vehicle's OBD system before
attempting a repair. In fact, the
repair community has been using
the kind of information provided
by OBD systems to diagnose and
repair vehicles for more than twen-
ty years. The fact that these sys-
tems and the information they pro-
vide has been standardized since
MY1996 makes using this informa-
tion easier while leading to more
accurate and quicker diagnoses and
more cost-effective repairs than
possible with earlier OBD systems.
Can the OBD system
be repaired, deacti-
vated, or modified?
The rule of thumb when it comes
to emissions-related vehicle repair
is that any modification that
changes the vehicle from a certified
configuration to a non-certified
configuration is considered tamper-
ing: this applies to both vehicle
owners and repair facilities and is
therefore a Federal offense.
Replacing a catalyst with a straight
pipe is one traditional example of
tampering. Likewise, overriding the
OBD system through the use of
high-tech defeat devices, non-certi-
fied computer chips, etc., would
also be considered tampering. The
OBD system may, however, be
repaired back to its original certi-
fied configuration with certified
"performance chips" or appropriate
aftermarket parts.
Will the repair
be covered by
warranty?
Warranty coverage varies depend-
ing on components and individual
manufacturer warranty provisions.
In most cases, however, responding
sooner rather than later is likely to
minimize the individual owner's
repair liability. The CAA requires
an 8 year or 80,000 mile warranty
on the major emissions control
components such as the catalytic
converter, and a 2 year or 24,000
mile warranty on other emissions
control components.
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