PackCheck is a program that will verify that all the files in your Content.ggpk file have the correct hashes.
From time to time we might ask you to run PackCheck and give us the output.
PackCheck is found in your Path of Exile install directory (by default "C:\Program Files\Grinding Gear Games\Path of Exile").
To use PackCheck:
1) Navigate to your Path of Exile install directory.
2) Right click PackCheck.exe and select Run as Administrator
3) Wait for the tool to complete. It can take a long time, and will print "Done." when finished.
4) If the tool reports any errors then please copy and paste the output into your bug report.
5) Run Path of Exile again. Any files that were corrupted should now repatch.