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How do I diagnosis a intermittent rich condition on a 96 altima obd II scan values for fuel trim are 20 25

may be knock sensor or temperature sensor fault

Right up my street. Your altima may be experiencing leaking fuel pressure regulator and fuel is going into the engine by the vacuum hose or leaky fuel injectors. Either is easy to find, pull pressure regulator vacuum hose it's at the end of the fuel rail see any gas? cap off the nipple at the intake and drive and re scan to see if the trim drops. Next you may be able to remove the complete fuel rail with all the injectors on it held down with two 12 mm bolts that hold the rail to the intake be cautious there's two black plastic heat insulator washers that can get lost do this over concrete disconnect the injector wires fish the fuel rail out there are four injector tip seals do not loose any of them. it's good to wash the area before starting dis assembly. Disconnect the distributor plug to kill the spark. With the rail raised up crank the engine some seconds and wait and watch for drips from the injectors one or two drips in a minute is OK dribbling isn't