United States Air and Radiation EPA 420-F-97-003
Environmental Protection May 1997
Agency
Office of Mobile Sources
vxEPA Environmental
Fact Sheet
Frequently Asked Questions
About On-Board Diagnostics
The Environmental Protection Agency (EPA) has regulations in place
establishing requirements for on-board diagnostic (OBD) systems on
light-duty vehicles and light-duty trucks beginning with the 1994 model
year. The purpose of the OBD system is to assure proper emission
control system operation for the vehicle's lifetime by monitoring
emission-related components and systems for deterioration and
malfunction.
What is OBD And How Does It Work?
By the early 1980's, numerous vehicles were using electronics and on-
board computers to control many of the engine's control systems, such as
fuel and ignition. Vehicle manufacturers had to develop ways to diagnose
problems generated by the new electronic hardware found under the
hood. Thus, the first OBD systems were developed by auto manufactur-
ers in the early 1980's as electronic systems replaced mechanical sys-
tems.
The engines in today's vehicles are largely electronically controlled.
Sensors and actuators sense the operation of specific components (e.g.,
the oxygen sensor) and actuate others (e.g., the fuel injectors) to maintain
optimal engine control. An on-board computer, known sometimes as a
"powertrain control module" or an "engine control unit," controls all of
these systems. With proper software, the on-board computer is capable of
monitoring all of the sensors and actuators to determine whether they
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are working as intended. It can detect a
malfunction or deterioration of the various
sensors and actuators, usually well before
the driver becomes aware of the problem
through a loss in vehicle performance or
driveability. The sensors and actuators,
along with the diagnostic software in the
on-board computer, make up what is called
"the OBD system."
What Is The Connection
Between OBD And Vehicle
Emissions?
The purpose of the OBD system is to
assure proper emission control system
operation for the vehicle's lifetime by
monitoring emission-related components
and systems for deterioration and malfunc-
tion. There are circumstances under which
the vehicle computer will detect a system
problem before the driver notices a
driveability problem. Furthermore, OBD
can detect problems that may not be notice-
able upon visual inspection because many
component failures that impact emissions
can be electrical or even chemical in
nature. By detecting these emission-related
failures and alerting the driver to the need
for potential repair, EPA hopes that ve-
hicles will be properly repaired before
emissions become a problem.
How Does OBD Inform Drivers
Of Problems?
When the OBD system determines that a
problem exists, a corresponding "Diagnos-
tic Trouble Code" is stored in the
computer's memory. The computer also
illuminates a dashboard light indicating
"Service Engine Soon" or "Check Engine"
or displays an engine symbol. This light,
usually yellow in color, serves to inform
the driver that a problem has been detected
and vehicle service is needed. When the car
is delivered to the repair shop, a service
technician can quickly retrieve the stored
diagnostic trouble codes from the computer
memory of the vehicle using newly devel-
oped diagnostic tools. Since the diagnostic
trouble codes will specifically identify the
problem, the service technician can more
quickly and accurately make the proper
repair.
It is important to note that an illuminated
dashboard light, as described here, is
intended to inform the driver of the need
for service, NOT of the need to stop the
vehicle. However, service should be
sought as soon as possible. Drivers also
may wish to consult a repair shop or their
vehicle owner's manual for further guid-
ance.
Why Does The Dashboard Light
Blink Or Flash?
Under certain conditions, the dashboard
light will blink or flash. This indicates a
rather severe level of engine misfire. When
this occurs, the driver should reduce speed
and seek service as soon as possible.
Severe engine misfire over only a short
period of time can seriously damage
emission control system components,
especially the catalytic converter, which is
typically the most expensive to replace.
Drivers should also consult their vehicle
owner's manual for manufacturer specific
information.
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How Can The Dashboard Light
Be Turned Off?
After fixing the problem, the service
technician will the turn off the dashboard
light. There are also situations under which
the vehicle's OBD system can turn off the
dashboard light automatically if the condi-
tions that caused a problem are no longer
present. If the OBD system evaluates a
component or system three consecutive
times and no longer detects the initial
problem, the dashboard light will turn off
automatically. As a result, drivers may see
the dashboard light turn on and then turn
off.
For example, if the gas cap is not properly
tightened after refueling, the OBD system
can detect the vapor leak that exists from
the cap not being completely tightened. If
the gas cap is subsequently tightened, the
dashboard light should be extinguished
within a few days. This is not an indica-
tion of a faulty OBD system. In this ex-
ample, the OBD system has properly
diagnosed the problem and accordingly
alerted the driver by illuminating of the
dashboard light.
How Does OBD Help The
Environment?
The intent of OBD systems is to assure
proper emission system operation of each
and every vehicle and light truck for its
lifetime by monitoring emission-related
components and systems for malfunction
and/or deterioration. An important aspect
of OBD is its ability to notify the driver of
a problem before the vehicle's emissions
have increased significantly. If the vehicle
is taken to a repair shop in a timely fashion,
it can be properly repaired before any
significant emission increase occurs. OBD
systems will also provide automobile
manufacturers with valuable feedback from
their customers' vehicles that can be used
to improve vehicle and emission control
system designs.
How Does OBD Help
Consumers?
OBD systems are designed to alert drivers
when something in the emission control
system begins to deteriorate or fails.
Early diagnosis followed by timely repair
can often prevent more costly repairs on
both emission control systems and other
vehicle systems that may affect vehicle
performance such as fuel economy. For
example, a poorly performing spark plug
can cause the engine to misfire, a condition
sometimes unnoticed by the driver. This
engine misfire can, in turn, quickly degrade
the performance of the catalytic converter.
With OBD detection of the engine misfire,
the driver would be faced with a relatively
inexpensive spark plug repair. However,
without OBD detection, the driver could be
faced with an expensive catalytic converter
repair in addition to the spark plug repair.
In addition, manufacturers have increased
incentive to build a higher quality vehicle
with better performance, reduced emis-
sions, and more efficient powertrains to
prevent problems that can lead to OBD
detection. OBD systems will also provide
far more information than ever before to
help auto technicians diagnose and prop-
erly repair vehicles during their first visit to
the repair shop, saving time and money for
consumers.
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Are OBD-Related Repairs
Covered By Warranty?
Federal law requires that the emission
control systems on 1995 and later model
year vehicles be warranted for 2 years or
24,000 miles. Many auto makers provide
extended warranty coverage beyond what
is currently required by federal law. Federal
law also requires that the on-board com-
puter and the catalytic converter on 1995
and later model year vehicles be warranted
for 8 years or 80,000 miles.
Can Anyone Service An OBD-
Related Problem?
Only qualified, trained technicians
equipped with the newest diagnostic and
repair equipment should conduct OBD
related service. All dealerships and inde-
pendent repair shops should have qualified
personnel for conducting OBD related
repairs. Vehicle owners should ask their
dealer and independent repair shops if their
technicians have received proper training
and have access to the necessary equipment
to properly service OBD equipped ve-
hicles.
Will Aftermarket Parts Work
With OBD?
Most aftermarket parts should work with
OBD systems, but there is no guarantee. It
is the responsibility of aftermarket parts
manufacturers to ensure that their parts
work properly with the vehicle for which
they are designed. This is even more true
for OBD. The OBD regulations have
required manufacturers to devise technolo-
gies and monitoring strategies that didn't
previously exist. However, EPA is confi-
dent that aftermarket part manufacturers
who do a thorough job of replicating
original equipment manufacturer parts and
those who carefully develop speciality
parts will be able to produce parts that
work with the OBD system.
For More Information:
For more information on federal on-board
diagnostic regulations please contact:
Holly Pugliese
U.S. EPA
Office of Mobile sources
2565 Plymouth Road
Ann Arbor, MI 48105
Phone:
E-mail:
pugliese.holly@epamail.epa.gov
Or, visit the Office of Mobile Sources
World Wide Web Home Page at:
http ://www. epa. gov/OMS WWW/
and follow the path: "Automobiles" "I/M
& OBD" "On-Board Diagnostics Files."
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