Use the drain valve under the fuel filter/water separator housing to drain all the water inside it. First, set a drain pan under the drain hose, then pull the drain valve handle to the OPEN position without the engine running, till all water and impurities are clear and clean fuel drains from the drain hose. Before replacing the drain valve, fuel heater element or Water-In-Fuel (WIF) sensor, fully drain the housing and throw away the mixture as instructed. Start with the fuel filter replacement by using a tool to unscrew the cap at the hex center, discard the O-ring and lift back the locking fingers to remove the filter from the cap. When replacing the WIF sensor, first disconnect the power cord, loosen the sensor by unscrewing, inspect the sealing ring and wipe off the probes if they are dirty. Unplug the electric wires from the fuel temperature sensor housing, remove the filter and the screws holding the heater element, pry the wiring connector from the housing and take off the heater from the filter. Before you change the drain valve, disconnect the hose, remove all the screws from the mounting bracket and take the drain valve and O-rings out together. After placing the tank cap on a surface covered with lint-free wipes, clean the filter housing and replace the cap. Next, fill the housing with clean diesel only until the top of the rubber bellows is present, snap the filter into place, put in a new O-ring, lightly oil the O-ring ring with clean diesel oil, insert the filter and tank cap into the housing and tighten the cap to 25 ft. lbs. torque. To install the WIF sensor, put in a new O-ring seal, coat it with clean diesel oil, set it in place with a torque wrench at 2-3 Nm (15-20 in. lbs.) and connect the electrical connector. Set the fuel heater element into the filter housing, install a new O-ring to the electrical connector, apply diesel oil, push the connector into the housing, set the temperature sensor housing with screws, attach the electrical connector and finally fit the fuel filter. For the drain valve, change the O-rings, use clean diesel oil, fit the valve in place, apply the mounting screws and tighten them to 3-5 Nm (30-40 in. lbs.), link the hose for fuel drain and finish by fitting the fuel filter. Bring the engine to life and see if you spot any leaks.