Hi,
My Nissan Altima 2003 (140K) has P0420 code. I tried to clear that code several times but keep getting it back after driving for couple days. The car drives well. I did not notice any changes in the performance. My mechanic advised me to replace exhaust manifold as it appeared to be the most common reason for that code. I purchased original used one but unfortunately the same code is back after driving 25 miles. I need some advice. Thank you very much.