The only purpose of your Nissan valve cover gaskets and seals is to hold oil in the engine's valve galley. Once these polymer components lose their ability to contain the oil, they will need to be replaced or oil can then cover the entire engine block along with the sensors that can then malfunction. Many Nissan owners that used to do their own maintenance on their vehicles in the past remember replacing the valve cover gasket and seals each time they adjusted the valve train. This was a periodical maintenance procedure that had to be done every 5,000 to 10,000 miles if following the manufacturer's recommendations. This procedure is no longer necessary since Nissan began using hydraulic lifters in the valve train assembly. The need for adjustments to the valve clearance is now automatic. Today the only reason for the replacement of the valve cover gasket and seals is if the Nissan engine has to be taken apart; for example, when a head gasket has to be replaced, PCV system needs work, or there is an oil leak from it. In either case a new set of Nissan valve cover gaskets and seals will have to be placed in position on the engine. The most difficult part of replacing the valve cover gasket and seals is the removal of all the old gasket material from the surface of the valve cover and cylinder head. Both of these contact points have to be entirely clean of old material along with oil and dirt. This makes it possible for the new seal to enclose these components and prevent any oil from leaking out. On most Nissan engines a new set of valve cover gasket and seals can be put into place in less than 2 hours.