That pesky Check Engine
Light, the “Malfunction
Indicator Lamp” or “MIL” for
short, is on because your
vehicle’s computer selfdiagnosed
a fault that could
cause your vehicle’s emissions
to increase. It doesn’t
necessarily mean your vehicle
is polluting or has a serious
problem – but it might. So the
light is on to let you know
something is amiss that needs
your attention.
Trouble is, you don’t know
WHAT the trouble might be. It
might be something that could
lead to a break-down
or cause expensive engine
damage, or it might be
something minor like a loose
gas cap (yes, the on-board
diagnostics on 1996 and newer
cars can even detect a loose,
missing or leaking gas cap).
There’s no way to know what
the problem is without talking
to your vehicle’s computer.
How do you do that? By
plugging a diagnostic scantool
into your vehicle’s diagnostic
connector, usually found under
the dash near the steering
column. These tools unlock the
secrets that have turned on your
Check Engine Light and tell you
the nature of the problem.
Yet these tools are only the
messenger. It’s up to you to
combine information from the
tool with knowledge of how
the electronic systems in your
engine work to make an
accurate diagnosis.
This booklet will give you the
information you need to
understand how to diagnose
and repair the most common
problems in today’s cars, light
trucks and SUVs. Whether you
perform the repairs yourself, or
use the information to better
understand what your mechanic
is doing, it pays to understand
what is going on.
Don’t be a Parts Swapper
The number one mistake
made by do-it-yourself
mechanics is “parts-swapping”.
This expensive practice leads to
the replacement of hundreds of
dollars of perfectly good parts.
At best, a parts-swapper wastes
time and money. At its worst, a
parts-swapper creates new
problems where none existed.
Armed with the proper tool and
the information in this booklet,
you will be prepared to tackle
the most common problems
found on today’s vehicles,
without being a parts-swapper.
The On-board
Diagnostic System - OBD II
When a fault is detected, the
onboard diagnostics system
(OBD II) records a “diagnostic
trouble code” (DTC) in the
computer’s memory. The code
number corresponds to a
particular type of fault. The
code might not tell you which
component has failed or why,
but it will tell you which
emission control system or
sensor circuit the fault is in, or
that your engine is misfiring or
running rich (too much fuel)
Today’s cars are
controlled by a
Powertrain Control
Module (PCM), your
engine’s computer.
If you want to repair
or modify your car’s
performance, you
need a scantool to
communicate with
the PCM.
or lean (not enough fuel).
When a basic code reader is
plugged into your vehicle’s
diagnostic connector, it will
display any diagnostic trouble
codes that are found. The least
expensive code readers just
give you a number while the
better ones also give you the
definition of the code, too.
You can read codes and get an
idea of what’s wrong – and
you can even clear the codes
from the computer’s memory
to turn out the MIL lamp (at
least temporarily). But a code
by itself isn’t the whole story.
Take a code P0171 or P0174.
These are codes that indicate
your engine is running lean. A
lean air/fuel mixture can be
caused by any number of
things, and may cause
symptoms such as a rough idle,
hesitation or stumble when
accelerating, hard starting, a
loss of power or an emissions
failure. Okay, so your engine is
running lean. Now what?
The next step up the
diagnostic ladder is to use a
scantool that can actually
provide some real diagnostic
information beyond basic code
numbers. What kind of
information are we talking
about? Real time sensor data.
This is the meat and potatoes
of onboard diagnostics and is
the data that real technicians
rely on to diagnose and repair
today’s vehicles. Some scantools
allow you to create a virtual
dashboard to monitor DTCs real
time parameters in a graph,
gauge, or tables shown in figure 1.
The diagnostic connector on
your vehicle can provide a
wealth of information, things
like engine speed, engine load,
air flow, ignition timing,
coolant temperature, inlet air
temperature, throttle position,
sensor voltages, what’s going
on with your engine’s fuel
management system, the
status of various switches and
devices, and more – in fact, up
to several hundred different
readings are available on some
cars depending on the make
and model year.
Diagnostic Scantools:
• Saves you money
• Saves you time
• Keeps your mechanic honest
Knowing what’s causing your
problem gives you the option
to either fix the problem
yourself or to take it to a
professional for repairs. If you
choose the latter, you won’t
be going in blind. You’ll be
armed with data that can save
diagnostic time and hopefully
some of your money.
In many instances, the Check
Engine Light is on because a
sensor has failed and needs to
be replaced. Some sensors are
relatively inexpensive and well
within the abilities of a do-ityourselfer
to replace. Some
sensors, on the other hand,
are quite expensive. A mass
airflow sensor might cost a
couple hundred dollars or
more. So you want to be sure
of your diagnosis before you
replace such an expensive part
– especially since most dealers
or parts stores won’t take
returns on electronic parts
once they’ve been installed.
With these things in mind,
let’s look at some common
problems
you’re apt to
encounter:
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