Retrieving the Sensor Codes From Your Vehicle"s On-Board Computer

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Do you know what to do when your Check Engine light comes on? When this happens, it usually means a sensor, actuator, electronic control module (ECM), or similar high-tech part within your vehicle's computer controlled system has failed.
If you follow my instructions, it will not be very difficult.
The computer generates a trouble code that tells you which part to test for failure.
During the process of retrieving trouble codes, you may be asked to disconnect the battery.
When the battery is disconnected, the electronic control module goes through a "relearn'' process that temporarily affects the vehicle's performance.
Normal performance returns after a short period of operation.
Each manufacturer has a different way of retrieving trouble codes.
You will need a service manual for your make and model.
Most sensors are simple to replace.
Once you have obtained the trouble code from the computer, proceed with testing the faulty part indicated.
To do this, consult the service manual for your make and model.
Before attempting to conduct the test, check the following components to eliminate them as the source of the problem: Battery - low voltage - low water level - battery terminal corrosion.
Distributor cap - cracks - carbon tracking.
Fuel filter - clogged filter.
Ignition Rotor - burnt electrode.
Spark Plugs - oil fouled - broken insulator - improper gap.
Spark Plug Wires - broken ignition wire - split plug boot or damaged terminals.
Vacuum hose - cracks - disconnected hose.
Once you've established that the performance problem is not being caused by the parts above, you should proceed with checking for trouble codes within your vehicle's on-board computer.
These are the Items you will need to check trouble codes of early models of GM cars you need: Code key, ten mega-ohm digital volt ohm meter, jumper wire, a nine volt battery, a piece of paper and a pencil, and a service manual for your make and model vehicle.
To access the on-board computer, locate the assembly line diagnostic link and remove the cover.
On most of General Motors models it is located under the dash on the driver's side.
You can use a code key or a jumper wire and insert it into the A and B ports of the ALDL terminal.
Turn the vehicle's ignition on but DO NOT START THE ENGINE.
To do so could cause severe damage to the electronic control module.
Watch for the Check Engine light to flash Code 12, showing that you have correctly accessed the computer.
The codes will flash one flash for the digit 1 and two flashes for the digit 2.
Code 12 shows three times.
If there is no problem, Code 12 will continue to flash.
If there is a problem, one or more trouble code will flash.
Watch for the codes to flash in the same banner as code 12.
For example, Code 33 is three flashes a pause, and three flashes.
After you've read the trouble codes once, wait for the cycle to begin again.
This time, write down the codes as they flash.
Once you have obtained the trouble codes, turn off the ignition switch.
The ignition switch must be turned off before the code key or jumper wire is removed to prevent system damage.
Then remove the code key or jumper wire.
Now that you have all the codes from your on-board computer you can move on to test them.
Testing each sensor will be different on each make and model car.
Check your manual for your make and model vehicle for instructions.
This is how you retrieve trouble codes from most GM vehicles.
Fords, Chryslers.
Most foreign makes and models are retrieved in a similar way.
You can now move on to testing some of the sensors that you found trouble codes on.
The computer-controlled part failure that each trouble code indicates is the number of similar diagnostic tests you can perform to confirm the part failure.
I will explain step-by-step how to do each test in my next article.
© 2010 Jesse Vibbert
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