Failed with Error 15: Cache Read Failure and Error 13 Permission denied, data cache needs to be flushed

If you are getting an error message about needing to flush your cache files (error ## varies):

 Failed with error ##: Cache File Corrupted
 The game data cache needs to be flushed in order to run.
 Did you wish to do this now? [OK] [Cancel]

There are several steps that can fix this error if flushing the cache via the [OK] button doesn't work:

Automatic Update needs to be enabled

The first thing to try is to go through all the installed games and make sure automatic update is enabled, a number of players have reported that this option was disabled and when they enabled it the game finished updating and the problem was solved.

You can check this via Steam > Games <right-click on your game> then click on "properties" and there is the option.

You can also try disabling the automatic update and then reabling automatic update which seems to fix this for another another group of players.

Try Deleting the Local Content

If these don't help, then the next step is to right-click on the game giving you a problem, select properties, then delete the local content for that game.

You'll have to re-download the content for that game but this should fix this problem for now.

If you have another machine with a working install or a backup of the GCF files then you can just exit Steam and copy/restore the entire set of GCF files over to the machine having the problem.

Sometimes the bad cache isn't the immediate cache for the game that is running (if more then one game is installed). For example, if CZ is having problems then you might have to delete local content for CS to fix the problem.

Last thing to try: Delete all the GCF files

If all the above fails, then the last solution is to exit from Steam and then go delete all of the Game and Common GCF files in the SteamApps directory (last ditch) and then re-download the Game content. If CS will run but not CZ then you don't need to delete the CS GCF file.

You don't need to re-install Steam just the restore the problematic cache file.

Chances are only one cache file got messed up it's just hard to tell which one.