Engine start-up I kept getting the code P0420?

I have a 2008 Nissan Altima I kept getting the code p0420 which resulted in me getting another transmission (used) got that put in I can tell the difference got new coils and spark plugs put in around the month of November also got a new catalytic converter put in front and back also. Now I keeping getting the code saying it's my downstream o2 sensor I bought 2 new 02 sensors and had them put in also got a code for crankshaft sensor and I bought that new also and had that put in I keeping getting the code for the 02 sensor even after replacing it. Every time I start my car up it's extremely loud and you can hear my car running a mile away where do u think that noise is from I thought it was coming from the engine but it's not sound like its coming from the back. Every time I start my car up its constantly cranks and cranks and I can hear it try and catch and try and turnover and most of the time it does and sometimes it will sound like a coughing effect and then turns off what do you think the problem is and why I am I still getting the code for m02 sensor after I already replaced both?