How do you use a code scanner?

With the car off, connect the scanner to the diagnostic port. Usually located just below the steering wheel, but some are hidden by a cover. Start the car and read the code. Do not connect or disconnect the scanner with the car on. google search the result or look it up in a Haynes or Chiltons manual to see what it is.

If you cant find the port, or still don't know how to do this go to any auto parts store like Autozone or Oreillys, they will help you out for free and look up the code for you.

If you want to clear the codes just to turn the check engine light off and the scanner does not have that option, disconnect your battery for 10 seconds, but do not do this before an emissions test. The vehicle has to do a "drive cycle" before it will either flag a new fault and illuminate the check engine light or pass the emissions test.

This is why its a good idea to have a scanner to verify a used car you are thinking about buying passed all the diagnostic tests. A shady dealership or individual selling the car may have cleared the fault just to turn off the light knowing it wont come back on until the car has been driven through a specific set of circumstances, after the sale is finalized.