Chapter 7 Computer Control Fundamentals 139
OBD II ECM Diagnostic Testing
The ECM in an OBD II system performs diagnostic
tests on various systems during operation. The ECM moni-
tors all system components during normal operation. This
is called passive testing. In addition, the ECM in an OBD II
system can perform active testing. In an active test, the
ECM orders a particular system or component to perform a
specific function while the ECM monitors performance. An
active test is usually performed in the event a component or
system fails a passive test, such as returning an out-of-range
reading. The third test is called an intrusive test. Intrusive
testing is a type of active test that can affect vehicle perfor-
mance or emissions. Many active and intrusive tests can be
performed by the technician using a scan tool.
Trouble Codes
OBD II systems use a five-digit, alphanumeric trouble
code, Figure 7-37. There are two general categories of
trouble codes. The first category consists of codes assigned
by the Society of Automotive Engineers. A comprehensive
list of these codes is located in Appendix B of this text. The
second category consists of manufacturer-specific codes.
These codes are exclusive to a manufacturer’s particular
vehicle control system. Look in the appropriate service
manual for these codes. Within these two general catego-
ries, there are four levels of diagnostic trouble codes:
p Type A codes are emissions related and will illuminate
the malfunction indicator light. In the case of misfire
or fuel trim DTCs, the ECM will flash the malfunction
indicator light whenever driving conditions cause this
code to be set in memory.
p Type B codes are also emissions related. However, the
ECM will illuminate the malfunction indicator light
only when this type of code appears on two consecu-
tive warm-ups.
p
Type C codes are not emissions related. They will not
illuminate the malfunction indicator light; however,
they will store a DTC and illuminate a service lamp
or, on a vehicle equipped with a driver’s information
center, the service message.
p
Type D codes are also not emissions related. They will
store a DTC, but will not illuminate any malfunction
indicator lights.
Malfunction Indicator Light
If a DTC is present, the vehicle’s malfunction indicator
light (MIL) will illuminate to notify the driver that there is a
problem. This light is usually amber colored and may also
be called the service engine soon, check engine, power
loss, sensor, or PGM-FI light.
This light may show MIL or the silhouette of an engine
when illuminated. The light will illuminate when a com-
ponent becomes defective, as on OBD I systems, but also
when the proper air fuel ratio is not being maintained or
when another output is not within specifications. The light
can illuminate in one of two ways: on steady or flashing.
MIL On Steady
If the OBD II system detects a problem that does not
have the potential to damage the catalytic converter, the
MIL will steadily illuminate. This indicates that the system
should be checked as soon as possible. While the vehicle
is driveable, prolonged operation with the MIL on could
cause damage to the converter or other vehicle emission
controls, as well as reduce fuel economy and driveability.
The MIL will not immediately turn off if the problem corrects
itself. OBD II systems require at least three trip sequences
(start, warm-up, and stop) before the MIL will turn off after
a problem corrects itself.
MIL Flashing
A flashing MIL means that an ongoing engine misfire
or other serious problem has occurred. Under severe mis-
fire conditions, the computer may shut off the fuel injector
and spark to a misfiring cylinder to protect the catalytic
converter. The driver may notice a loss of power. In many
cases, the light will stop flashing when the vehicle is
restarted or the condition is no longer present. Even if the
light does stop flashing, it will often remain illuminated. In
any case, the vehicle should be brought to a service facility
for diagnosis and repair as soon as possible. Some manu-
facturers recommend that a vehicle with a flashing MIL not
be driven, but be towed to the nearest service facility.
In some cases, the MIL will flash because of an
extremely low fuel level allowing air to enter the injection
system, a missing fuel filler cap, or excess fuel entering
the evaporative control system during refueling. In these
cases, the problem is temporary and the OBD II system will
cease flashing the MIL after a period of engine operation.
However, if there is any doubt as to the cause of the flash-
ing MIL, the vehicle should be checked.
Figure 7-37. OBD II codes are a series of five digit letters and
numbers. The letter and first number is called the alphanumeric
discriminator. The second number indicates the nature of the
code. The last two numbers compose the fault designator
and indicates which sensor or circuit is at fault and the type of
problem.
Example: P0173 Fuel Trim Malfunction (Bank 2)
B=Body
C=Chasis
P=Powertrain
U=Network
Fault designator
0=SAE
1=Manufacturer
1=Fuel and air metering
2=Fuel and air metering (fuel system)
3=Ignition system or misfire
4=Auxiliary emissions control
5=Vehicle speed, idle control and auxiliary inputs
6=Computer and auxiliary outputs
7=Transmission
8=Transmission
9 and 0=Catergory to be determined