All of the Ford Focus models manufactured before 2005 use the same process for replacing spark plugs. To maintain a well-oiled machine, replace its spark plugs every 60,000 to 100,000 miles.
Find the spark plug wires or ignition coils on the engine. For models made before 2000, these may be located along the sides. For models made in 2000 or later, the spark plugs are along the front and back of the engine. If the spark plugs run along the front and back, stand to the side of the Focus and look along the back of the engine to find the spark plug wires.
Grab a hold of the wire boot or the top of the ignition coil and twist to release the wire from the spark plug. Pull back on the boot to remove the spark plug wire or ignition coil. Complete the replacement process for each spark plug before removing any other spark plug wires.
Slide the socket over the spark plug and turn to the left to unscrew the spark plug from the cylinder well. Depending on the Focus model's year, an extension may be needed to reach the plugs.
Remove any corrosion or dust from the well or boot. Gently brush around the sides of both pieces with the wire brush.
Spray silicone into the well, on the threads of the spark plug and into the wire boot. Silicone spray keeps the spark plug from sticking in the well of the engine.
Place the new spark plug into the well and turn it to the right by hand, if possible. Some spark plugs are placed in areas where turning by hand is impossible. Slide the socket and extender onto the spark plug and turn with the extender gently to thread the spark plug into the well.
Test the spark plug tension with the tension tester. Most spark plugs require 8 to 15 foot pounds. Check with the spark plug manufacturer or check the spark plug's packaging for the correct tension.
Replace the ignition coil or spark plug wire and move to the next spark plug. Continue the process until all of the spark plugs are replaced.
Allow the engine to cool down completely before beginning this process. Removing pieces from a warm engine causes cylinder block damage.