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The so-called VAC Authentication Error is an error where some software*, or faulty hardware**, or setting on your computer conflicts with Valve's Anti-Cheat (VAC). In some cases, internet connection issues, like an unstable connection, can also cause this error.

As a result you get kicked from the VAC protected server you are currently on in the middle of a match (in any game mode, be it Competitive, Casual, Deathmatch, Demolition or Arms Race). You are then unable to reconnect to that server, which -- in Competitive game mode -- will result in a so-called Cooldown for you.

Since Valve does not reveal anything about the exact functionality of their anti-cheat tool, which is running on all their official game servers, nobody can explain to you how and why this conflict is occuring.

So, before you start troubleshooting it, take a deep breath and calm your nerves. You might be up for a bumpy ride.

*) IMPORTANT NOTE: Only programs that alter your system or launch at Windows startup and run as background applications or as services can be a possible software-related causes for this issue.

**) After a year of having this error re-occur repeatedly (in 2015) and also experiencing severe lag spikes earlier this year (2016), which almost always were followed by a VAC disconnect, I built in a new PSU -- no more lag spikes, no more VAC Authentication Error. More on that in the next section.

The so-called VAC Authentication Error is a known issue.

The official thread containing all known official as well as unofficial software- and system-related solutions can be found at the SteamPowered forum: [FIX] An issue with your computer is blocking the VAC system. If the suggested solutions in the entry post on Page 1 don't work for you, check out Page 12 regarding Windows Updates. (Note: If you are running Windows 10, make sure you deactivate DVR in your Xbox app's settings, as it is known to conflict with Steam games.)

An important thing is missing in the list of solutions on Page 1, though:

• First of all, your motherboard must be capable of using DEP. To determine whether it is, enter the BIOS at startup. Depending on your motherboard's manufacturer, the option to enable this support may be labeled "Data Execution Prevention," "XD," "Execute Disable," or "NX."
  
  
• If your motherboard supports DEP, good. Now check in Windows if it's activated:
  
  Control Panel > System and Security > System > Advanced system settings > Performance > Settings > Data Execution Prevention > Turn on DEP... > OK
  
  

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On a forum, I read about Powershell on Windows 10 being associated with the error:

Your recent disconnects by VAC have been caused by running Powershell alongside the game. Powershell is a core function of Windows which gives you access to various services for the purpose of task automation and configuration management. It's typically used by power users or users running scripts.

You can stop Powershell through your task manager by pressing CTRL+Shift+Esc and checking the 'Processes' tab. Once here look for powershell.exe and select 'End Process'.

If this doesn't help, we recommend reaching out to Windows Support for in-depth help with disabling Powershell to avoid being disconnected by VAC in the future. Because of its functionality in Windows we don't provide in-depth support for disabling Powershell at Steam Support.

You will also want to make sure that your RAM is not faulty, as this could trigger a false alert since VAC is constantly reading your computer's memory. Furthermore, I highly recommend executing Steam with administrator rights, as for some reason not doing so can lead to all sorts of errors related to connecting to Valve's servers.

In addition, check your hardware for possible power issues. Maybe your PSU cable is faulty or doesn't connect 100% safely to your PSU; or maybe something isn't wired properly, like some fan for instance. Voltage fluctuations (including power outages!) can trigger this problem as well as affect your system's functionality.

If none of the solutions in this section worked for you, read on...

Check and, if necessary, repair vital Windows files using Windows' System File Checker:

1. Run the Command Prompt (cmd.exe) as Administrator.
   
   
2. Type the following command line to initiate a full scan of your system files:
   
   sfc /scannow
   
   
3. Press Enter and wait for the process to finish (which will take a while). Don't end it prematurely!
   
   
4. Restart your computer if System File Checker did actually repair any files. (Note: System File Checker may or may not prompt you to restart, but even if it doesn't, you should restart anyway.)

I keep reading posts blaming CCleaner[www.piriform.com] for this error. This is only true when you don't restart your computer after using CCleaner. I therefore recommend you only use it at the end of the day, before shutting down your PC.

Since CCleaner is a great tool in the hands of an experienced PC user, I would like to discuss its settings with you:

1. Avoid having it run in the background; you should deactivate any automated features and run it manually (usually once a week is enough). The following functions need to be unchecked:
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2. Make sure everything under "Advanced" (in the Windows tab) is left unchecked, otherwise it can lead to a variety of issues for inexperienced users:
   
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3. I repeat, do not use it before you launch Steam. Always restart your PC once you are done with CCleaner's cleaning operation.

If you follow these simple rules, you can be 100% certain that CCleaner won't pose a problem and can further narrow down the cause for this issue.

As for Avast[www.avast.com] (as well as any other antivirus), make sure all Steam-related processes* are placed in the list of exceptions:

Settings > Components > Behavior Shield > Exclusions

Do that and you can rest assured that it is not the cause for your problem with VAC.

The same goes for your firewall settings; make sure all Steam-related processes* are granted permission to access the internet.

*) ...are as follows:

• C:\Program Files (x86)\Common Files\Steam\SteamService.exe
  
• ..\Steam\GameOverlayUI.exe
  
• ..\Steam\Steam.exe
  
• ..\Steam\steamerrorreporter.exe
  
• ..\Steam\steamerrorreporter64.exe
  
• ..\Steam\bin\cef\cef.win7\steamwebhelper.exe (folder name may vary per OS version)
  
• ..\Steam\bin\x64launcher.exe
  
• ..\Steam\streaming_client.exe
  
• ..\Steam\SteamApps\common\Counter-Strike Global Offensive\csgo.exe

.. = the location of your Steam installation folder

As mentioned earlier, hardware issues are among the possible causes for the VAC Authentication Error, and there are countless hardware issues that your system could experience.

It would be insane (and impossible) to attempt listing all of them in this guide. Solving issues regarding your system's power supply alone would fill several pages. Therefore, I recommend using Google to find online guides on specific issues.

• For a general overview of your system specs, I recommend installing Speccy[www.piriform.com]. It also displays the temperatures of your hardware components. (If you wish to monitor your system for possible CPU, GPU or HDD overheating detection, install HWMonitor[www.cpuid.com].)
  
  
• Flash your BIOS to the latest available version.
  Reset your BIOS settings for test purposes.
  Replace your BIOS battery if you suspect a problem (a faulty battery can cause your system to run erroneous).
  
  
• Check your HDD[support.microsoft.com] for errors/recover faulty sectors.
  
  
• Test your RAM[www.memtest86.com]. (Note: when you run the test, which takes several hours, you need to do it one RAM module at a time to achieve accurate results.)
  
  
• Check your CPU with CPU-Z[www.cpuid.com] and run a CPU Stability Test[www.majorgeeks.com].

You can always contact Steam Support. At first, you will be redirected to their official support page. After that, they will ask you to send them your MSINFO32 report. Once they went through all the information this file contains, they might suggest you deactivate (or uninstall) certain programs known to be potentially responsible for the issue. I say "potentially responsible", because they might not be in your case and will most likely not lead to a permanent solution. Why is that?

Unfortunately, there is no universal fix. Many programs or certain hardware constellations can potentially trigger the issue, the cause is different for everyone and depends on your system.

Basically, if none of the known fixes work for you, the best option to solve this is to start from scratch:

1. Re-install Windows (deactivate auto-updates!).
   
   
2. Install only a handful of programs/updates at a time and test for a couple of days (best do so in other match-making game modes like Casual, so you won't get another Cooldown if it kicks you off the server again).
   
   
3. Make a Windows backup* (I recommend Acronis TrueImage 2015[www.amazon.com]), then install the next program(s) and test again. And so on.

It's the only safe way to solve this once and for all, as long as the error is not hardware-related (if you got this far and the error still persists, you can safely assume that it is an issue with either your hardware or your internet connection). Steam Support only offers the basic solutions which do not work for the majority of people. If you read the many forum posts regarding this issue (on the Steam Community forums, on SteamPowered, Reddit and what not), you will come to the same conclusion.

So good luck getting to the bottom of this problem and getting it solved. :)

*) Always make system backups (system partition + system reserved partition) once a month so you can always go back to a state where everything worked fine. This error has the annoying habit of returning every once in a while if you weren't able to locate and eliminate its cause.

Apart from the error-related aforementioned fixes, you can try basic ones that help with all sorts of Steam- and Steam game-related errors, as provided by Valve's Steam Support:

• Exit Steam.
  
• Click Start > Run (Windows Key + R)
  
• Type the following command:
  
  "C:\Program Files (x86)\Steam\bin\SteamService.exe" /repair
  
  If you have installed Steam to another path, please replace C:\Program Files (x86)\Steam with the correct path. This command requires administrator privileges and may take a few minutes.
  
  
• Launch Steam and test the issue again.

• Right-click the game's name in your Steam games list
  
• Select "Properties" > "Local Files" > "Verify Integrity of Game Files..."

Sometimes, for some reason, a Steam game insists that your connection was unreliable. While this is a different issue, there is still a possibility of this interfering with your "safety" connection to VAC.

To fix this issue:

1. Exit Steam
   
2. Launch Steam with Administrator privileges:
   Right-click Steam.exe or its shortcut > Properties > Compatibility > check "Run as administrator"
   
3. Launch the Command Prompt (cmd.exe) and execute the following three commands:
   
   • ipconfig /release
     
   • ipconfig /renew
     
   • ipconfig /flushdns
4. Exit Command Prompt. The problem should be fixed. If not, try rebooting the system before launching Steam again.

Make sure nobody else is playing the game on your network (including WiFi) that you may not know about. Also, if there are several users in your household, make sure you configure a unique port for each computer connected to your router.
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» Don't forget to check out my other guides as well