The spark plugs on a 2003 Mitsubishi Lancer are the "spark" component of the fuel/spark/air combination required by an internal combustion engine, but over time, they can wear out. Replacing them is fairly easy to do, but you do need to make sure to properly gap the plugs, otherwise you'll be doing more harm to your engine than good. This should take about an hour to do properly.
Pop the hood and use the 3/8-inch ratchet and socket to unbolt the spark plug wire cover on the center of the engine if your car has one.
Pull up the far right spark plug wire by gripping it at the boot and pulling up with a twisting motion. Then remove the stock spark plug using the 3/8-inch ratchet, extension and spark plug socket. Discard the old spark plug.
Gap the replacement spark plugs using the spark plug gapping tool and referring to the specifications listed in the owner's manual for the car. To do this, slide the gapping tool between the ground electrode (the bent piece of metal at the end of the plug) and the center electrode (the central part of the spark plug), then make sure that the distance between the two is the same as listed in the owner's manual. If not, adjust the electrodes as necessary.
Install the replacement plug into the cylinder head using the 3/8-inch ratchet, extension and spark plug socket. Push the spark plug wire boot back onto the spark plug until you hear a popping sound, indicating the it's locked in place.
Repeat the process with the next spark plug over, working systematically, one plug at a time, so you minimize the chance of confusing the wires.
