Here’s the full story with the 02 through 05 Altimas.
Since the QR25DE motor doesn’t use an EGR system, it phases the intake and exhaust valves to lower the NOx temps.
The problem is, that this causes exhaust gasses to be sucked back into the combustion chamber from the exhaust manifold.
For a while, Nissan used a bad cat converter material, and as the miles piled on, the catalyst material would flake off the converter, and be sucked back into the combustion chamber, and slowly sand blast away at the cylinder walls, eventually increasing the bore size until the engine started consuming engine oil on an alarming level.
Some engines can suck down 2 quarts of oil in a week, driving on the highway, 50 miles each day, in some reported cases.
So, if the original engine was replaced with a new/manufactured one due to massive oil consumption issues, but the converter wasn’t replaced, it should be replaced TODAY. If the installed engine was a used one, with greater than 80k miles, compression and leak down tests should be done to determine the health of the replacement motor, and the converter replaced to prevent any further damage.
BC.