Section 185 (Wheel And Tire System): Tire: Inspection: Notes

WARNING: This page is about a different car, the 2004 Subaru Forester. However, it is still accessible from the selected car via links, so may be relevant.

Take stone, glass, nail etc. off the tread groove.
Replace the tire:
CAUTION: When replacing a tire, make sure to use only the same size, construction and load range as originally installed.
1. When large crack on the side wall, damage or crack on tread is found.
2. When the "tread wear indicator" appears as a solid band across the tread.
3. If a crack is found on the tire valve, replace the tire valve.
  
  Fig 1: Identifying Tire Tread
  Courtesy of SUBARU OF AMERICA, INC.