Once you cars computer has recorded a engine or transmission trouble code and the repairs have been made the code will remain in the memory of the computer until it is cleared. This record is used as a safety so the problem can still be found in case the electrical power is lost to the computer in situations such as when your car's battery goes dead. All cars built after 1996 have this code storage capability and will need a code reader to clear the codes once the check engine light has come on. This system is called an OBD2 engine management system.
You will need a code reader tool which you can get from Amazon or the local auto parts store starting at about $30.00 (US).
Locate the ALDL connector (16 pin D shaped computer connector) which is under the dash on the driver side. Connect the code reader and then turn the key to the on position without starting the engine. Using the command buttons on the reader tool follow the prompt and choose the "erase codes" or "clear codes" option from the tool's LED screen and hit enter. The tool will ask you if you are sure you want to erase the codes, push yes. The tool will now go into your cars computer and remove the codes from its memory, this takes about 15 seconds. The tool will respond by displaying a no codes present on the screen.
The reader will then present an option to recheck the system to make sure the codes have been cleared and rerun the code reading process. The code reader or scanner will then display a pass message reading if no problems are detected. To be clear this pass state might be temporary if a problem still exists which will only show up after a while of driving. The check engine light will come back on immediately if a hard code problem is detected.
A hard code is when the ignition key is first turned to the on position and engine ran for a few seconds compared to a regular code which can take a few cycles of the car being ran and then turned off and then ran again before it shows up. After completing the code run sequence you should get a "Pass" message on the screen letting you know the codes have been successfully erased. The check engine light will come back on again if the computer detects the same or a different problem within about 50 miles of driving.
To disconnect the scan tool turn the ignition key to the off position and then disconnect the tool. Do not remove the tool before turning the key off because to can interrupt the computer's communication ability and make the next code gathering incomplete or create a no communication problem.
On all cars made before 1996 you can remove the trouble codes by simply disconnecting the negative battery cable end and wait for 15 seconds, then reconnect the battery. You will still need to drive the car for about 50 miles to be sure the code doesn't not come back on in which case the codes will need to be gathered again to see what is wrong.