In order to scan your car for codes, you must first locate the diagnostic connector.  It will be under the
drivers side dash. Here is an example of its location. Your  vehicle may be in a little different spot.

If you do not have a car code reader, many of the large national parts store chains  will scan your cars
computer for free.
On todays newer vehicles (1996-and up, OBDII), the diagnostic terminal is what  
powers up the scan tool. If the scan tool does not power up after plugging in to the  
connector, you will need to
check fuses. Different vehicles use different fuses to  
power this connector.
After reading the codes(s) and the short description, you will need to use a diagnostic  
chart to pinpoint the problem. Just because the scanner shows, lets say, a code for the  
intake air temperature sensor, does not necessarily mean that the sensor is bad.
The most basic scanners will just read trouble codes from the Powertrain Control  Module (PCM). A little higher
priced scanners will read codes, and give some live data  streams. The high end scanners will do this, and read
other computers such as Body  Control Module(BCM), Anti-lock Brake module(ABS), Airbag module(SIR), Door  
modules, 4-wheel drive modules, etc. These will also be able to control devices in a  given circuit to go along with
diagnostic charts, and run preprogrammed test, and  re-program replacement computers.
Location of the ALDL connector to check for GM computer trouble codes.
Scan tool installed to read computer codes.
12-9-10
The heads up display on a 05
Grand Prix won't come on.
Comments
12-11-10
Check all the fuses. Make sure the switch is turned on.
1-6-11
i am trying to find the meaning of 2 codes on my 2005 trailblazer
they are P0641 and also 2138
The first is a 5 volt reference problem. Either a
wiring problem, or a faulty
electronic throttle,
accelerator pedal, coolant fan speed sensor, AC
pressure sensor, fuel tank pressure sensor or
Secondary air Injection valve circuit. The other
relates to the accelerator pedal position sensors
1 - 2 incorrect correlation.
2-4-11
my vehicle will reverse but it wont drive.
4-9-11
trying to find out what c1295 code is on a chevy van adventure.
8-17-11
what does error only mean on the code reader
10-3-11
I'm trying to find out why when I press on the gas I get this squealing
noise.
Sounds like the scanner is not linking up properly
with the car computer.
10-6-11
code p1870 for 1996 GMC jimmy.
P1870 is transmission component slipping. Either
your have a bad trans or a solenoid problem.
Squealing is normally caused buy a bad belt or
possibly a pulley.
10-20-11
I have a code P2432 on an 07 Trail blazer it is  idling rough and has
knocking sound when revved up and stalls.
10-20-11
I cannot find code for P0455,i think it has something to do with my
tranny shifting hard from 1st to 2nd.2004 cavalier.
Code P0455 is a large EVAP system leak.
Possibly a broken wire at the vent valve under the
rear of your Chevy Cavalier.
12-7-11
My 2001
Chevy Malibu won't start. It turns over but won't go. It also
says CAL ERR on the radio and the dash wont light up to tell me the
millage.
Too many things to list. Do you have spark, fuel
pressure, injector signal, theft light etc. CAL ERR
means your radio is locked. Odometer not lighting
up is most likely a bad gauges cluster assembly.
1-2-12
i have a 2005 Chrysler Sebring and everything is going on  my
odometer reading keeps blinking my check engine light,brake light,
battery light slippery pavement light and my abs n brake light is all
on and when i drive the flicker.... Please help me establish what i
could be facing
1-12-12
trans off light and abs lights won't go off on my 2000
Monte Carlo.
1-12-12
Never heard of a transmission off light?
1-20-12
2002 Toyota Camry , Trac off lights come on after putting car into
drive won't go off.
2-28-12
My 02
Chevy Impala check engine light stays on then it blinks
sometimes when j reach over a certain speed it keeps saying misfire
on cylinder 4 but I've changed the coil the plug and the wire but it
still comes up misfire?
Well, the last things are a sticking fuel injector or
an internal engine problem like a broken valve
spring or rocker arm stud that has pulled out of
the cylinder head (on the 3.4L engine only)
3-4-12
I have a 1993 Pontiac Bonneville,with a 3.8l v6 supercharger, the
car in pretty good shape especially the interior. I am getting a
service engine light, can I electrically check to see what the problem
may be? Also since it is supercharged not anyone can work on it, is
there a book or video i can purchase to enlighten me on the care of
my car? Thanks.
Being supercharged doesn't limit a shop or
mechanic from being able to work on it. It isn't
anything special on your car. The check engine
light should really be checked with a scanner, but
since it is OBD1, you can purchase a small tool
that looks like a two pronged key to insert into the
diagnostic port to flash the codes over the check
engine light.
3-12-12
I have and 2001 Chevy impala and my Anti lock is on my brakes.
Also my service traction light, tracker light is on. Every time I drive
then brake its start to grind and then completely stop
3-13-12
Where can I find the crankshaft position sensor on my 1996 Pontiac
Sunfire 2.4L 5 speed Manuel transmission.
The crank sensor is near the starter. Best to view
it from under the car. Just one bolt olds it on.
Common is a bad front wheel speed sensor.
4-24-12
Replaced IAC Valve in a 2000 malibu. It idles rough car bucks back
and forth. Also it makes a high pitch squeal from IAC valve when
driving. Any help would be appreciated.
Not sure how you know there is a high pitch
squeal from the IAC while you are driving ! The
hood is closed and you are inside the car I would
assume, because you are driving ! You have a
vacuum leak.
I get intermittent idle issues on my 2005 ford explorer sport trac -
sometimes to where it dies, and I can't start for about 5 minutes.  A
few times (4 or 5) the check engine light came on, with code p1235 -
I've replaced my Fuel filter and fuel pump driver module - and am
having a hard time recreating the issue.  It seems to do it when it
wants to.. Any tips/tricks?  I'm using a pinpoint test for the problem,
but can't figure out how to do the following (the first step):

Access the PCM and monitor the FPM PID.
Observe the FPM PID for an indication of a concern while carrying
out the following:
Shake, wiggle, and bend the FPC circuit between FPDM and the
PCM
Lightly tap on the FPDM to simulate road shock  

Do you have any suggestions?
Sounds like a throttle position sensor problem.
I have a GM quick card with two prongs that fits into data link module
you then count the flashes on the dash , which terminals does this
go in.
Depending on the year of your car, i think it is terminals 2 and 7.

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How To Scan Car Computer Codes
jack abascal said:
tried to have parts store scan for codes on my 1998 lincoln town car, and it could not read any it just said hit enter after all information was put in for
car.

Bill said:
@jack abascal, So it is either a problem with the scanner or a problem with your car. Try a different store, or better yet, a repair shop. No way to tell if
it is a problem with that parts stores scanner or your car without trying another one.