(P-Codes). Now, basic terms are standard-
ized and all generic codes will share a
common format and terminology that the
manufacturers and the Society of Auto-
motive Engineers (SAE) designed. You
will be glad to know that as your car gets
smarter, it will be easier for you to keep
track of what is going on under the hood.
16
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! Spe-
cial testing abilities are permanently pro-
grammed into the computer by factory
engineers. These tests check the compo-
nents connected to the computer which
are used for (typically): fuel delivery, idle
speed control, spark timing, emission sys-
tems, and transmission shifting. Mechan-
ics have used these tests for years. Now
you can do the same thing by using your
Actron scan tool!
Engine computers perform special tests
The engine computer runs the special
tests, depending on the manufacturer,
engine, model year, etc. There is no “uni-
versal” test that is the same for all vehicles.
The tests examine INPUTS (electrical sig-
nals going INTO the computer) and OUT-
PUTS (electrical signals coming OUT of
the computer), as well as internal calcula-
tions 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 test results
The test results are stored by using code
numbers, usually called “trouble codes”
or “diagnostic codes.” For example, a code
22 might mean “throttle position sensor
signal voltage is too low.” Code meanings
are a part of your scan tool’s software—all
you have to do is look them up! But since
code definitions vary with manufacturer,
model year, and engine, you may also
want to refer to a vehicle service manual
for additional information. These manu-
als are available from the manufacturer,
other publishers, or your local public li-
brary. See page 2 for more information on
ordering service manuals.
Read Trouble Codes with the scan tool
You can obtain trouble codes from the
engine computer memory by using the
scan tool. You can also monitor the opera-
tion of systems throughout the vehicle,
helping to pinpoint the system where there
may be a problem. Once you have read
the trouble codes, you can either:
• Have your vehicle professionally ser-
viced.
Or,
• Repair the vehicle yourself using the
trouble codes to locate the source of the
problem.
2-2 About Codes
Where do Trouble Codes come from and what are they for?
17
Trouble Codes and Diagnostics help
you fix the problem
To find the cause of the problem yourself,
you need to perform special test proce-
dures called “diagnostics.” These proce-
dures are in the vehicle service manual,
and your scan tool makes it easy to locate
and diagnose malfunctioning systems.
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 a bad
bulb or light socket? Are there problems
with the wiring or wall switch? Maybe
there is no power coming into the house!
As you can see, there are many possible
causes. The diagnostics are 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.
Actron makes it easy to fix computer-
controlled vehicles
Using the Actron scan tool to obtain
trouble codes 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!
2-3 When to Read Codes
Many vehicles have a “Malfunction Indicator Lamp” or MIL, which has been referred to
as a “Check Engine” light in the past. With the advent of OBD-II, all engine trouble lights
are now called “Malfunction Indicator Lamps” or MIL.
Use the Malfunction Indicator Lamp to tell you when trouble codes have been
stored in memory:
• “Check Engine”
• “Service Engine Soon”
• “Service Engine Now”
• marked with a small engine picture or
diagram
The Malfunction Indicator Lamp is nor-
mally OFF when the engine is RUNNING.
NOTE: The Malfunction Indicator Lamp
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.
About the Malfunction Indicator Lamp
Malfunction Indicator Lamp:
normal operation
The engine computer turns the Malfunction
Indicator Lamp on and off as needed. This
dashboard message is either amber or
red and labeled:
