What do you do when the dreaded Check Engine light comes on in
your car? You could take to a mechanic, who will probably charge
you an arm and leg just to diagnose the problem. Or you could
invest in a scan tool, which will pay for itself by letting you
diagnose the problem. What is a scan tool? I'll get to that in
a bit, but first we need to consider On Board Diagnostics.

Cars built since the early 1980s have On Board Diagnostics, or OBD.
This means that the car has a computer which continually monitors
sensors and operating conditions. The computer adjusts the fuel
and ignition systems to optimize performance and minimize pollution.
The computer also knows when something is off-normal, and then it
generates a trouble code. This is what causes the Check Engine
light to come on.

Generation I OBD vehicles, or OBD-I, were built from 1982 to 1995.
Generation II OBD vehicles, or OBD-II, were built from 1996 to
present (as of this writing, late 2008). OBD-II vehicles are more
complex than OBD-I cars, and have more sensors. They are also more
expensive to repair. A fuel pump for my OBD-I GM vehicles costs
about $40. A fuel pump for an OBD-II vehicle costs about $400, from
what I have heard, because of all the sensors in the assembly.
I plan to keep my OBD-I vehicles as long as I can. Apparently you
have to be rich to drive a newer car.

Sometime in the future there will be a third generation, OBD-III.
I have read some really scary, Orwellian Big Brother stuff about
OBD-III proposals. An OBD-III car may have radio frequency transmission
capability, such that the car's operating conditions are relayed
to a police car or monitoring station. That way the cops will know
for sure if you are speeding, or if you are wearing your seat belt,
or if your car is exceeding pollution standards. An OBD-III car
may also have to receive transmissions, perhaps from a police
officer who can remotely stop your car's engine. If this comes
to pass, I wouldn't drive an OBD-III car even if it were free.
I'd rather ride a bicycle, or a horse.

But, back to the scan tool, which is a way for you to learn
what is wrong when the Check Engine light illuminates the dashboard.
I am basing this explanation on OBD-I GM vehicles, since that is what
I am familiar with. There are two types of tools that can help you
diagnose what's wrong. The first is the plug-in code reader that
causes the Check Engine light to flash. You see, the Check Engine
light is not just an "idiot light." With the code reader, you count
the flashes to give you the trouble code. You plug the code reader
into the ALDL, or assembly line data link. Then turn the ignition key
to ON, but don't start the car. The Check Engine light will begin
to flash. You need to count the flashes. For GM vehicles, the first
code is always 12, which is flash - pause - flash - flash. The 12
code is flashed three times. Then, if there are trouble codes, they
are displayed. For example, code 32 is flash - flash - flash - pause -
flash - flash. Each trouble code is displayed three times. At the end of the
sequence, the 12 code is displayed again, three times. The 12 code
does not indicate a problem, it merely shows you the system is
working and starts and ends the code display sequence. So if you
get a trouble code, what does it mean? The code reader tool should
come with a book that tells you what the codes mean. An auto repair
manual, for example from Chilton or Haynes, will also give a listing
of the trouble codes. Some of these code readers will read ABS brake
trouble codes as well.

The other type of scan tool is the diagnostic scanner. It is more
expensive than the code reader, but also gives you more information.
This is a hand-held module with a cable that plugs into the ALDL. I
have the AutoXray scanner, and it has been extremely useful. It
also retrieves trouble codes, but in addition, it gives you detailed
information on the engine, fuel injection, and electrical systems.
It monitors, in real time while the car is running, variables such as
vehicle speed, engine rpm, oxygen sensor voltage, battery voltage,
coolant temperature, and outputs from other sensors, e.g, MAP and
throttle position sensors. My 1991 Chevy S10 has more monitored
variables than my 1984 Cavalier or my 1986 Celebrity. Some of these
scanners can interface with a personal computer to archive data.

Here's an example of how these tools are helpful. Several years
ago my S10 began to race at idle. Eventually it
kicked out a trouble code. I was able to read the code, low
coolant temperature, and verify with the AutoXray scanner that
the engine's computer thought the temperature was -40 degrees.
I then used a multimeter to measure the resistance across the
terminals of the coolant temperature sensor. It had failed open.
Since the computer thought the temperature was -40 degrees, it
was giving the engine way more gas than it needed, hence the high rpm.
All I had to do was replace the coolant temperature sensor with a
new one, and the S10 was back in business. (And to clear the code
in a GM OBD-I vehicle, simply disconnect the battery or pull the
fuse to the computer, or ECM. That way the trouble code won't
keep displaying after you have fixed the problem.)

Other essential tools for the do-it-yourself mechanic are a
digital multimeter and a repair manual.

Even if you don't want to do your own repairs, having a scan tool
can save you money. You can use it to check a used car you are
looking at buying to see if it has problems. You can use it to
keep mechanics from ripping you off. If the trouble code shows
a bad oxygen sensor, then the mechanic who is telling you that you
need a new transmission is lying.

Where can you buy scan tools? I have seen them at retailers such
as Sears, Wal-Mart, and Harbor Freight Tools. The JC Whitney catalog
and website has a good selection of them. An auto parts, speciality or
performance store should also carry them.