Actron CP9135 OBD II AutoScanner User Manual
Page 34
B-5
About OBD II
About the Malfunction Indicator Lamp (MIL)
All
OBD
II
compliant
vehicles
have
a
“Malfunction
Indicator
Lamp”
or
MIL.
In
the
past,
the
MIL
has
been
referred
to
as
a
“Check
Engine”
or
“Service
Engine
Soon”
light.
Normal
operation
The
engine
computer
turns
the
MIL
ON
and
OFF
as
needed.
This
dashboard
message
is
either
amber
or
red,
and
maybe
labeled
with
“Check
Engine”,
“Service
Engine
Soon”,
“Service
Engine
Now”,
or
marked
with
a
small
engine
picture
or
diagram
The
MIL
is
normally
OFF
when
the
engine
is
RUNNING.
NOTE: The
MIL
will
turn
on
when
the
ignition
key
is
in
ON
position,
but
the
engine
is
OFF
prior
to
starting
the
vehicle.
This
is
a
normal
test
of
all
the
dashboard
message
lights.
Problem
spotted
If
the
MIL
does
not
come
on
during
this
test,
you
may
have
an
electrical
problem
which
needs
repair.
Refer
to
the
“Diagnostic
Circuit
Check”
steps
of
your
vehicle
service
manual.
Current
problem
When
the
MIL
remains
ON
after
the
engine
is
RUNNING,
the
computer
sees
a
problem
that
does
not
go
away
(known
as
a
“current”
failure).
The
light
will
stay
on
as
long
as
the
problem
is
present
and
a
trouble
code
is
stored
in
the
computer’s
memory.
Use
the
OBD
II
AutoScanner
at
the
earliest
convenient
time
to
obtain
codes.
Intermittent problem
When
the
MIL
comes
ON
and
then
goes
OFF
while
the
engine
is
RUNNING,
the
computer
saw
a
problem,
but
the
problem
went
away
(known
as
an
“intermittent”
failure).
Though
the
MIL
went
OFF
because
the
problem
went
away,
the
code
stays
in
memory.
Use
the
OBD
II
AutoScanner
at
the
earliest
convenient
time
to
obtain
codes.
NOTE: The
computer
will
automatically
erase
these
codes
after
repeated
restarts
if
the
problem
does
not
return.
Poorly
running
engine,
no
MIL
Most
likely,
this
condition
is
not
due
to
computer
system
failures,
but
reading
codes
can
still
be
useful
as
part
of
a
basic
troubleshooting
procedure.
Check
wiring
and
bulb
for
“Check
Engine”
light
failures.
Refer
to
vehicle
service
manual
for
additional
diagnostic
information.
On
OBD
II
vehicles,
the
MIL
also
signals
an
emissions-control
related
failure.
The
vehicle
may
not
run
any
differently,
but
the
OBD
II
system
is
designed
to
note
very
small
changes
in
the
engine’s
operation
which
could
lead
to
emissions
damage
or
failure.