Question

I am getting P0420, P0725 and P0335 codes in my 2002 Altima ... what could be the possible cause?

Answer 1

p0335 and p0725 are for Crankshaft Position Sensor whichs needs to be replaced (if your battery is fine replace the sensor)

here is a youtube video that will help

Nissan Altima Camshaft and Crankshaft Position Sensor Replacement Procedure Video

p0420 catalyst converter is most likely bad if you dont have any other engine problems.