Actron CP9135 OBD II AutoScanner User Manual
Page 33
About OBD II
B-4
About Diagnostic Trouble Codes (DTCs)
Engine
computers
can
find
problems
The
computer
systems
in
today’s
vehicles
do
more
than
control
engine
operations—they
can
help
you
find
problems,
too!
Special
testing
abilities
are
permanently
programmed
into
the
computer
by
engineers.
These
tests
check
the
components
connected
to
the
computer
which
are
used
for
(typically):
fuel
delivery,
idle
speed
control,
spark
timing,
emission
systems,
and
transmission
shifting.
Mechanics
have
used
these
tests
for
years.
Now
you
can
do
the
same
thing
by
using
your
OBD
II
AutoScanner!
Engine
computers
perform
special
tests
The
engine
computer
runs
the
special
tests,
depending
on
the
manufacturer,
engine,
model
year,
etc.
There
is
no
“universal”
test
that
is
the
same
for
all
vehicles.
The
tests
examine
INPUTS
(electrical
signals
going
INTO
the
computer)
and
OUTPUTS
(electrical
signals
coming
OUT
of
the
computer),
as
well
as
internal
calculations
made
by
the
computer.
Input
signals
which
have
“incorrect”
values,
or
output
circuits
which
do
not
operate
properly
are
noted
by
the
test
program
and
the
results
are
stored
in
the
computer’s
memory.
These
tests
are
important.
The
computer
cannot
control
the
engine
properly
if
it
has
incorrect
input
information
or
faulty
output
circuits.
Code
numbers
reveal
malfunctions
Malfunctions
are
stored
by
using
code
numbers,
usually
called
“diagnostic
trouble
codes”
or
“DTCs.”
For
example,
a
code
P0122
might
mean
“throttle
position sensor
signal
voltage
is
too
low.”
Generic
code
meanings
are
a
part
of
your
OBD
II
AutoScanner’s
software.
Manufacturer
specific
DTCs
will
require
the
use
of
a
vehicle
service
manual.
See
page
2-5
for
more
information
on
ordering
service
manuals or on the enclosed product CD.
Read
trouble
codes
To
find
the
cause
of
the
problem
yourself,
you
need
to
perform
special
tests
called
“diagnostics.”
These
procedures
are
in
the
vehicle’s
service
manual.
There
are
many
possible
causes
for
any
problem.
For
example,
suppose
you
turned
on
a
wall
switch
in
your
home
and
the
ceiling
light
did
not
turn
on.
Is
it
the
bulb,
light
socket,
wiring,
or
wall
switch?
Maybe
there
is
no
power
coming
into
the
house!
As
you
can
see,
there
are
many
possible
causes.
The
diagnostics
written
for
servicing
a
particular
trouble
code
take
into
account
all
the
possibilities.
If
you
follow
these
procedures,
you
should
be
able
to
find
the
problem
causing
the
code
and
fix
it
yourself.
Using
the
OBD
II
AutoScanner
is
fast
and
easy.
Trouble
codes
give
you
valuable
knowledge
-
whether
you
go
for
professional
service
or
do
it
yourself.
Now
that
you
know
what
trouble
codes
are
and
where
they
come
from,
you
are
well
on
your
way
to
fixing
today’s
computer-controlled
vehicles!
Once
you
have
read
the
DTCs,
you
can
either:
• Have
your
vehicle
professionally
serviced,
or
• Repair
the
vehicle
yourself
using
the
diagnostic
trouble
codes
to
help
locate
the
source
of
the
problem.
With
the
OBD
II
AutoScanner,
you
can
also
monitor
the
operation
of
systems
in
the
vehicle,
helping
to
pinpoint
the
system
where
there
may
be
a
problem.