The Ford Focus uses a belt driven water pump to circulate coolant -- a 50/50 mix of water and antifreeze -- through the engine. As the coolant moves through the engine it absorbs heat generated by the combustion process. Heat coolant is released to the air in the radiator as air is pulled across it's cooling fins by the electric fan. To keep the engine cool, the fans must work properly and the coolant must flow freely through the engine.
Allow the engine to cool completely. Visually inspect the radiator hoses for obvious splits and leaks that could cause coolant to be lost and overheating to occur. Verify the serpentine belt isn't missing. The water pump is driven by a single serpentine belt that also drives the alternator and power steering.
Check the coolant level in the expansion tank located on the passenger side of the engine compartment. The tank is clearly marked low and full. Add coolant as needed. Remove the pressure cap from the top of the expansion tank and attach the pressure tester to the expansion tank in its place.
Pump the pressure tester until the pressure on the tester's gauge measures the same as the pressure marked on the top of the pressure cap. Inspect all radiator and coolant hoses for leaks and replace any leaking hoses. Inspect the water pump for leaks behind the water pump pulley. Replace the water pump if it leaks behind the pulley.
Start the engine and allow it to run until the thermostat opens. The upper radiator hose will get hot when this happens. If the engine overheats and the upper hose remains cool -- replace the thermostat.
Observe the cooling fans. They should come on automatically shortly after the thermostat opens. If the fans do not come on and the engine overheats turn off the engine. Unplug the electrical connector for the cooling fan motor located on the bottom of the fan motor.
Check cooling fan operation using a test light to verify the power and ground when the engine is hot. Start the engine. Clip the test light to the negative battery terminal and probe the fan's electrical connector. One of the terminals in the connector should cause the test light to come on when it is touched with the probe indicating power is being supplied to the fan.
Switch the test light to the positive battery terminal and touch the terminals in the connector. The opposite terminal in the connector should cause the test light to come on, indicating ground is supplied to the fan. If power and ground are supplied and the fan doesn't come on, replace the fan motor.
Replace the cooling fan fuse if the previous test showed that power was not supplied to the fans. If ground was not supplied to the fans, replace the coolant sensor screwed into the cylinder head below the ignition coils on the driver's side of the engine.
Wear safety glasses and work gloves when working around a running engine to prevent serious injuries.