The belt tensioner on a 2002 Ford Focus applies tension to the serpentine drive belt to keep it running on the pulley path correctly. Without the tensioner, the belt would fly off in a matter of seconds, once the engine was started. The tensioner itself consists of a spring loaded arm and pulley. In some cases, the spring can break inside the arm or the bearings inside the pulley can crack. If either of these malfunctions plagues your Focus, you can have a new tensioner installed in about an hour.
Raise the bonnet and connect the belt tensioner tool to the belt tensioner.
Lever the belt tensioner to the side with the tensioner tool. Reach down with your free hand to move the belt out of the tensioner pulley's path. Once the belt is out of the way, release the tensioner. Remove the tensioner tool.
Use the socket set to unbolt the tensioner flange from the engine. Depending on how your Focus is equipped, you may need to unbolt and remove the idler pulley first to access one of the tensioner pulley's mounting bolts. The idler pulley can be removed by simply removing the bolt in the centre of the pulley with the socket set and moving the pulley away from the engine by hand.
Remove the old tensioner after the last bolt is out. Position the new tensioner in its place. It is best to compare the two briefly before installing the new one, just to make sure you were sold the correct replacement part.
Threads the bolts back in by hand to avoid cross threading them. Tighten them down with the socket set. If you removed the idler pulley in Step 3, reinstall it now in reverse of how you removed it.
Put the tensioner tool on the new tensioner. Move it aside so you can route the belt over the new tensioner's pulley. Release the pressure on the tensioner. Remove the tool. Start the engine and monitor the travel of the serpentine belt to make sure everything is working properly before driving the Focus.
If the old tensioner failed, you should inspect the serpentine belt and replace it if there are any signs of damage.