Question

2006 Nissan Altima Sedan with engine codes P0300 and P0420, and a hard start

Answer 1

There are many things that can fail and trigger that code. You could start by replacing the spark plugs and if possible, add a fuel additive. If the problem persists the next step is to perform a compression test.

P0300 2006 NISSAN ALTIMA SEDAN - Cylinder Misfire Detected Random Cylinders
Read more: https://www.autocodes.com/p0300_2006_nissan_altima_sedan.html

P0420 2006 NISSAN ALTIMA SEDAN - Catalyst System Efficiency Below Threshold Bank 1
Read more: https://www.autocodes.com/p0420_2006_nissan_altima_sedan.html