96 cadillac seville. changed intake due to leak at blow off flap. bad gasket. put on used intake. new intake gaskets. 6 months later i get a p0171 and p0174 code. bank 1 and 2 lean condition. i checked every vacuum line and hose and pcv valve. brake booster too. nothing seems bad. than i got a code p0131, O2 sensor. think these are related? the plenum is dry. no coolant passages to deal with. the intake gaskets are heavy molded plastic with silicone ribs. almost impossible to put in gaskets wrong. attach gaskets. lower intake. how hard is that? i know the torque is low as i cracked the original intake 2yrs ago. any ideas?