Spark plugs work with the engine's ignition coils to help fire and start your car. You should change the spark plugs in your Ford Focus car every 30,000 miles, especially if they are corroded or the electrodes are bent out of place. Unlike older cars, the 2005 Focus does not use individual spark plug wires with its spark plugs; instead, the individual ignition coils are mounted over the spark plugs.
Disconnect the car's negative battery cable.
Remove the individual ignition coil to access its spark plug--disconnect the coil's electrical connector and remove the coil's mounting bolt with a wrench, then lift the coil out.
Blow away all dirt and debris from the spark plug's hole with compressed air.
Insert your wrench with a spark plug socket into the hole and turn it counterclockwise to remove the spark plug from the engine.
Coat the threads of your replacement spark plugs with an anti-seize compound using a small, thin brush, taking care not to get any of the compound on the plug's porcelain surface or electrodes.
Thread the spark plug into its hole by hand, then tighten it using the wrench and spark plug socket, turning it clockwise.
Install the ignition coil back in place with its mounting bolt and connect the electrical connector.
Repeat all of the above steps for any more spark plugs that need changing.
Reconnect the battery cable once you have replaced all the plugs you need to.
Platinum spark plugs can last for up to 60,000 miles. Don't cross-thread the spark plugs or you'll damage the coil. If you place a small rubber hose over the spark plug and use it to install the plug in the hole, the hose will start slipping if the plug is cross-threading.