1999 nissan altima code p0420

1999 nissan altima engine 2,4 162000 mileages. First time appear the cod p0420. I checked exhaust system for leak and replaced rear oxygen sensor.Engine worked good and cod did not eppear. But after 250miles appears cod p 0171. I checked intake system and replaced lower intake manifold- to cylinder-head gasket,because the engine missed at idle speed. After it engine runs good, but again appears cod p 0420. Who can help me. Thank You.
PO420 is low catalyst efficiency. Almost always meaning the cat is dead. P0171 is system lean. A poor running engine can, and will, destroy a cat. Need to address the P0171, first if it is still present. (a dead cat won't cause a drivability issue, but a plugged one will).
Loren
SW Washington The PO420 is a bad Catalytic Converter. It took a while to reset because it's a marginal call plus is takes a whole bunch of factors to qualify for that code and it takes a while for them all to come up.