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Understanding On-Board Diagnostics (OBD)
The purpose of the OBD system is to ensure proper emission control
system operation for the life of the vehicle by monitoring emissions
related components and systems for deterioration and malfunction.
Federal regulations establish requirements for on-board diagnostic
(OBD) systems on light duty vehicles and light duty trucks.
OBD turns on a special warning light on the dashboard called a Malfunction
Indicator Lamp (MIL) when it finds a fault that may cause high engine
or evaporative emissions.
When the MIL is on, it means that something is wrong with the emissions
system that requires repair.
If the MIL is flashing, prompt service is required.
Manufacturers cover emission control systems for varying periods,
and your vehicle repairs may be covered by warranty.
The following are typical examples of a MIL:
Or a standard symbol can be used, as in this example:
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- Q. What is
OBD and how does it work?
- A. Since the early 1980's, many
vehicles have used electronics and on-board computers to control
many of the engine 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, auto manufacturers developed the first on-board diagnostic
(OBD) systems as electronic systems replaced mechanical systems.
A more sophisticated on-board diagnostic system was eventually
developed, and the second-generation on-board diagnostic system
became required for most 1996 and newer passenger vehicles, light-duty
trucks and similar vehicles.
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” (PCM) or an “engine
control module” (ECM), controls all of these systems. The on-board
computer is capable of monitoring all of the sensors and actuators
to determine whether they 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 drivability. The sensors and actuators,
along with the diagnostic software in the on-board computer, make
up what is called “the OBD system.”
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- Q. What is
the connection between OBD and vehicle emissions?
- A. OBD is an emissions control system.
Older emission tests that collect or sample the exhaust produced
by a vehicle identify vehicles that are already excessive polluters.
However, OBD is a shift to pollution prevention. OBD can identify
problem(s) with the emissions control system before the vehicle
becomes an excessive polluter, allowing time to repair the vehicle
before emissions increase. Left un-repaired, further damage can
occur and emissions will increase.
There are circumstances under which the vehicle computer will detect
a system problem before the driver notices a drivability problem.
Furthermore, OBD can detect problems that may not be noticeable
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, vehicles can be properly repaired before
emissions become a problem.
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- Q. How does
OBD inform drivers of problems?
- A. When the OBD system determines
that a problem exists, a corresponding
“Diagnostic Trouble Code” (DTC) is stored in the computer's memory
and a special lamp on the dashboard called a Malfunction Indicator
Lamp (MIL) is illuminated. This lamp is reserved for emissions
problems only and cannot be used for other failures. Automobiles
use a variety of warning lamps to notify drivers of different conditions.
In the case of an emissions MIL, either the phrase “Service
Engine Soon” or “Check Engine” is used or an engine
symbol is displayed. 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. The stored diagnostic trouble code(s)
will help the service technician diagnose and repair the problem.
It is important to note that an illuminated MIL 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 guidance.
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- Q. Why does
the Malfunction Indicator Lamp (MIL) blink or flash?
- A. Under certain conditions, the
MIL 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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- Q. How can
the Malfunction Indicator Lamp (MIL) be turned off?
- A. Service technicians can manually
turn off the MIL, but OBD systems are designed to automatically
turn off the MIL if the conditions that caused a problem are no
longer present. If the OBD system evaluates a component or system
three consecutive times (with some exceptions) and no longer detects
the initial problem, the MIL will turn off automatically.
As a result, drivers may see the MIL 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 MIL should be extinguished within a few days. This
is not an indication of a faulty OBD system. In this example, the
OBD system has properly diagnosed the problem and accordingly alerted
the driver by illuminating the MIL.
If the MIL is turned off manually without correction of the underlying
problem that caused the initial MIL illumination, the OBD system
will reset the readiness monitors (see “What
are readiness monitors?”), and begin the evaluation of
the emissions control systems. If the OBD system has not completed
the evaluation prior to the emissions test (see “How
do readiness monitors affect the automotive repair industry?”
and “How can readiness Monitors be set?”),
the vehicle will be rejected for “not ready”. If the OBD
system completes the evaluation and detects the problem again, the MIL
will re-illuminate, and the vehicle will fail the emissions test.
Therefore, simply turning off the MIL prior to the emissions test,
will NOT allow the vehicle to pass.
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- Q. What are
readiness monitors?
- A. 1996 and newer vehicles manufactured
for sale in the U.S. include software designed to indicate when
emissions control systems have a defect that may lead to elevated
emissions. These systems include readiness monitors that are used
to find out if emissions components have been evaluated. In other
words, if all monitors are set to ready, the emission
components have been tested.
These monitors are included in the OBD system so overall vehicle
condition can be easily assessed electronically at an emissions
test station, using a data link connected to the vehicle. To ensure
that the data provided to the test station reflects actual vehicle
condition, the vehicle computer reports whether or not the data
in the computer is current and valid. This check is referred to
as “readiness”, and the vehicle will be denied an emissions
test if it fails the readiness check.
A vehicle can have up to twelve monitors built into the OBD computer
system. The most common monitors are:
- Continuous
- Misfire
- Fuel System
- Comprehensive Components
- Non-Continuous
- Oxygen Sensor
- Heat Oxygen Sensor
- Catalyst Efficiency
- Evaporative Emissions System
- EGR System
- Secondary Air System
- PCV System
Depending on the vehicle and the scan tool being used, the language
referring to readiness conditions can vary. Some of the possible
nomenclature includes:
| If monitor is “ready” |
If monitor is “not ready” |
RDY
OK
Completed
Yes
Done
COMPL |
Not RDY
Inc
Not Completed
No
Pending
INCOMPL |
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- Q. How do
readiness monitors affect the automotive repair industry?
- A. In the past, technicians routinely
cleared codes – either before and/or after performing repairs
-- as a routine part of the repair process. Clearing codes resets
all monitors to a “not ready” status. Some monitors
are easily run during normal driving; however, others can take
a long time to run. This can cause problems if the vehicle has
to return to a test station for a re-test because vehicles can
not be tested if a sufficient number of monitors have not run.
Remember, vehicles are rejected when more than two non-continuous
readiness monitors are “not ready” for 1996 to 2000
model year vehicles, or when more than one non-continuous readiness
monitor is “not ready” for 2001 and newer model year
vehicles. Additionally, the catalyst monitor must be “ready” at
the time of any re-test after a vehicle failed an initial test
for any catalyst code.
By clearing codes during the repair process, shops and technicians
may find themselves in situations where the customer takes the
vehicle back to an emissions station for a re-test after repairs
have been made, but the vehicle is rejected at the station because
a sufficient number of monitors are not ready. Therefore, shops
and technicians should seriously reconsider the practice of clearing
codes on OBD equipped vehicles when that vehicle has to return
for an emissions re-test. Shops can protect their Repair Effectiveness
Index (REI) by making sure that a sufficient number of monitors
are “ready” and bringing vehicles back for a re-test
after repairs are complete. After the vehicle passes the re-test,
the vehicle can be returned to the customer.
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- Q. How can
readiness monitors be set?
- A. There are many reasons the monitors
could be set to “not ready”. One cause is routine maintenance.
For instance, if the battery is disconnected for any reason, the
monitors of most vehicles are reset. Also, a service technician
may have to reset them as part of the repair process. In these
cases, the car must be driven to reset the monitors. Some manufacturers
advertise driving procedures while others do not. The vehicle manufacturer
or a qualified service technician is the best source for this information.
Another cause is that there is a problem with the OBD system that
prevents one or more monitors from running. In these cases, a qualified
technician must diagnose and repair the problem before the monitor
will run.
Before any of these monitors set to “ready”, the components
must be operated in a specific manner designed by the manufacturer
that checks the performance of that particular emissions control
system. However, it is important to understand that setting a monitor
to “ready” does not ensure that the system is defect-free.
Completed readiness monitors only indicate that the applicable
component or system has been checked.
Three monitors on all OBD equipped vehicles have been designed
to continuously check for system defects. Because of this, monitors
report as complete at all times and will not cause a vehicle to
be rejected during a vehicle emissions test. These continuous monitors
evaluate the following emissions control systems in the vehicle:
- Misfire
- Fuel System
- Comprehensive Component
In order to set readiness for non-continuous monitors, the vehicle
must be prepared and driven in a specific manner. Monitors can
not be set using a scan tool, and the OBD system software must
see the driving conditions required to run the monitor in order
to complete the evaluation. These required driving conditions are
referred to as “drive cycles” and define the preconditions
and driving conditions necessary to set the readiness monitors.
Since many different readiness monitors may be present on a vehicle,
drive cycles can be designed to set all monitors present with one
cycle, or an individual drive cycle can be targeted at specific
monitors on the vehicle. It should be understood that both the
catalyst and evaporative monitors require more driving than other
monitor drive cycles.
Due to the complex interrelations between OBD system components
and monitors, it is possible for one defect to mask or “block” an
additional defect(s). Because of this, it is possible to perform
a valid repair on a vehicle only to have an additional defect identified
by the readiness monitors shortly after the repair .
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- Q. A special
note about the catalyst monitor?
- A. After a vehicle fails an OBD
test where any of the fault codes are for catalyst efficiency,
the catalyst monitor must be “ready” prior to a re-test.
Such vehicles presented for a re-test with the catalyst monitor “not
ready” will be rejected.
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- Q. How
does OBD help the environment?
- A. The intent of OBD systems is
to ensure proper emission system operation for each and every vehicle
and light truck during 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 also provide automobile manufacturers with
valuable feedback from their customers' vehicles that can be used
to improve vehicle and emission control system designs.
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- Q. How
does OBD help consumers?
- A. OBD systems are designed to alert
drivers when something in the emission control system begins to
deteriorate or fail.
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.
Furthermore, manufacturers have increased incentives to build higher-quality
vehicles with better performance, reduced emissions, and more efficient
powertrains to prevent problems that can lead to OBD detection.
OBD systems also provide far more information than ever before
to help auto technicians diagnose and properly repair vehicles
during their first visit to the repair shop, saving time and money
for consumers.
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- Q. Are
OBD-related repairs covered by warranty?
- A. 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 automakers provide
extended warranty coverage beyond what is currently required by
federal law. Federal law also requires that the on-board computer
and the catalytic converter on 1995 and later model year vehicles
be warranted for 8 years or 80,000 miles.
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- Q. Can
anyone service an OBD-related problem?
- A. Only qualified, trained technicians
equipped with proper diagnostic and repair equipment should conduct
OBD related service. Vehicle owners should ask dealers and independent
repair shops if their technicians have received proper training
and have access to the necessary equipment to properly service
OBD equipped vehicles. The Repair
Shop Report Card is an excellent resource to help select a
qualified repair shop.
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