Looked up this code and saw many issues this might be so I would like to narrow it down before going to a mechanic.
When I am driving and slowing to stop it is like something disengages with a small clunk and then re-engage as I begin to accelerate again.
This only happens while going very slowly. Otherwise the vehicle runs great with no rough idle at all.
Can anyone shed some light?
This code is pointing to an issue with the catalytic converter. This is not likely what you are hearing, there are several possible causes for the code, but the sound won't be there. I would check the engine and transmission mounts.
P0420 NISSAN - Catalyst System Efficiency Below Threshold Bank 1
Read more: https://www.autocodes.com/p0420_nissan.html