On-Board Diagnostics (OBD) For Vehicles - July 2009
WHAT IS OBD AND
HOW DOES IT WORK?
On-Board Diagnostic systems
(OBD) were developed in
the 1980s to help technicians
diagnose and service the
computerized engine systems
of modern vehicles. A new
generation of these systems is found in 1996 model
and newer vehicles. These new systems, regardless of
the type of vehicles, now monitor the same compo-nents,
use the same computer “language” and have
the same criteria for evaluating the systems and for
indicating the problems to the driver and the repair
The OBD system provides drivers with an early
warning of malfunctions through a dashboard “Check
Engine” light (also known as a Malfunction Indicator
Light or MIL). This early warning light not only helps
protect the environment, and helps consumers, by
identifying minor problems before they become
major repair bills.
HOW DOES ADEQ USE OBD?
When it is time for a vehicle equipped with OBD to
be tested for emissions, the technician at the test
facility simply connects a cable or remote wireless
connection to the vehicle’s computer connector. The
vehicle connector is usually found under the dash and
is known as the DLC (Diagnostic Link Connector).
Information stored in the vehicle computer is reported
to the emissions-test computer and analyzed. The
process is very quick and the report can be given to
the technician if necessary to help repair the vehicle.
WHY DOES OBD TESTING REPLACE
TAILPIPE TESTING ON NEWER VEHICLES?
The OBD II system is required in 1996 and newer
vehicles by federal law. The system consists of com-puter(
s) and sensors built into the vehicle to monitor
emission systems. While the tailpipe test looks at the
vehicle exhaust to determine if the vehicle is running
efficiently, OBD looks for broken or malfunctioning
components which are causing or will cause emissions
failure if not repaired. OBD often identifies failing
components long before tailpipe emissions would fail.
This, in turn, often allows for repair at a much lower
cost than a malfunctioning component that damages
the engine or catalytic converter.
HOW DOES THE DRIVER KNOW IF THERE IS
When the OBD system determines that a problem
exists, a corresponding “Diagnostic Trouble Code” is
stored in the computer memory. The computer also
illuminates the dashboard “Check Engine” light,
informing the driver that a problem exists, and that
service is needed. If the light is flashing, the problem
is serious and needs immediate attention.
WHAT IF THE MIL LIGHT COMES ON AND
THEN GOES OFF?
As the OBD system checks components, it may
encounter a system or component that is out of
tolerance. If this happens, the MIL light will be illumi-nated.
If the OBD system later finds the system to be
in tolerance, it will (over a period of time) turn the
lamp off. This does not indicate a problem with the
OBD system. Your repair technician can perform a
specific test to determine if the system is working
CAN THE “CHECK ENGINE” LIGHT BE
When the engine service is performed, the techni-cian
will turn off the “Check Engine” light. There are
also situations when the vehicle computer will turn off
the light. If the OBD system scans the component
three consecutive times or more, and no longer
detects the problem, the light will turn off automati-cally.
For example, if the gas cap is not properly tight-ened,
the light may go on. If subsequent fueling causes
the cap to be properly tightened, the light will go out
in a few days.
Publication Number: FS 08-13
WHAT DOES “NOT READY” MEAN?
Your vehicle’s computer reviews the status of the
emissions systems within the vehicle. After most
repairs, or if the battery is disconnected or replaced,
the status of all systems will be “not ready’ until the
vehicle is operated long enough to reset the monitors.
The vehicle should be operated after service for as
long as a week in order to reset the status to “Ready.”
If too many status monitors are “Not Ready” a vehicle
will be rejected from the test.
IS THE OBD REPAIR COVERED BY
Federal law requires that the emission control
systems on 1995 and newer model year vehicles be
warranted for a minimum of two years or 24,000
miles. Warranty coverage for the OBD computer and
catalytic converter (only) is extended to eight years or
80,000 miles for these vehicles. Most automakers
provide extended warranty coverage beyond that
required by law. Depending on the mileage and model
year of your vehicle, emissions repair may be covered
by the manufacturer. Consult your warranty or dealer.
WHAT IF THE “CHECK ENGINE” LIGHT IS
Some people mistakenly disconnect the “Check
Engine” light since they cannot detect any problems
with vehicle operation. The light is provided to alert
drivers to potential component failure. Proper repair
will save both time and money and reduce the
chance of emissions-test failure. The OBD system is
required on all 1996 and newer light-duty vehicles.
When the MIL light is disconnected, the vehicle
cannot pass emissions testing, is no longer protected
by warranty, and is in violation of state and
WHO CAN PERFORM OBD REPAIRS?
Anyone may do the repairs necessary for OBD
emissions failures. However, it is necessary to have a
diagnostic tool known as a “Scan Tool” to properly
query the onboard computer and to verify that the
repairs have been successful and the monitors reset.
We suggest that only a qualified repair technician
equipped with the necessary scan tool and training
perform emission related repairs on OBD-equipped
DO REPAIRS COST MORE BECAUSE OF OBD?
Because OBD serves as an early warning system
for the vehicle owner, repairs often cost less than
engine repair or a new catalytic converter.
Quite often repairs due to the “Check Engine” light
are limited to service or replacement of an individual
WHERE CAN I GET MORE INFORMATION?
There are two Web sites; www.vei.azdeq.gov and
www.myazcar.com can provide most of the information
you need to find an inspection station, repair facility,
test fees, exemptions and much more.
We have an On-Board Diagnostics (OBD) informa-tional
brochure that is also available. It can be found
on the ADEQ Web site www.azdeq.gov, click on the
heading “Brochures & Fact Sheets”.
For vehicle specific information, or for information
not covered in the above resources you may contact
Vehicle Emissions Inspection Program staff in Phoenix
at (602) 771-3950 or in Tucson at (520) 745-4536.
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