Nissan QR25DE Engine Problems: Seeing bluish smoke and Code PO420?

It seems as if all of these complaints are quite common with the Altima 2.5L (QR25DE) engine: code PO420, exhaust rattling, and seeing bluish smoke coming from your exhaust. All these symptoms can point to the same cause: a bad exhaust manifold/catalytic converter. The factory catalytic converter is designed to last the lifetime of the vehicle. …