Section 246 (Wheel And Tire System): Tire: Inspection: Notes

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

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