Valve Seat Contact

WARNING: This page is about a different car, the 2013 Nissan Armada. However, it is still accessible from the selected car via links, so may be relevant.

After confirming that the dimensions of the valve guides and valves are within specifications, perform this procedure.
Apply prussian blue (or white lead) onto the contacting surface of the valve seat to check the condition of the valve contact on the surface.
Check if the contact area band is continuous all around the circumference.
If not, grind to adjust the valve fit and check again. If the contacting surface still has NG conditions even after the re-check, replace the valve seat.

Fig 1: Identifying Proper And Improper Valve Seat Contact
Courtesy of NISSAN NORTH AMERICA, INC.