“Decompression failed” means your computer tried to unpack a compressed file and couldn’t finish the job. Most software, games, and updates are distributed as compressed archives to save bandwidth, and your system must expand them back to their original size during installation. When something interrupts or corrupts that process, the installer halts and throws a decompression error. The cause is usually a damaged download, a hardware issue, or a permissions problem on your system.
How Decompression Works (and Breaks)
When you download a game or application, the files are packed into a compressed archive that’s much smaller than the final installed product. During installation, your computer reads that archive, reconstructs the original data, and writes it to your hard drive. To make sure nothing went wrong, the installer runs an integrity check: it compares a short verification code embedded in the archive against a freshly calculated code from the unpacked data. If the two codes match, the file is intact. If they don’t, the process aborts immediately and you see a decompression error.
This verification system is called a cyclic redundancy check, or CRC. Every compressed archive includes CRC values attached to its data blocks. Even a single flipped bit in the file will cause the stored value and the computed value to diverge, and the installer has no way to know which version is correct. It simply stops and tells you something is wrong. That’s the “failed” part of the message.
What the Specific Error Codes Mean
If your error mentions “Unarc.dll,” you’re dealing with a common decompression library used by many game installers and repackaged software. The number after the error narrows down the problem:
- Code -1: A generic decompression failure. The archive couldn’t be unpacked, but the system can’t pinpoint why.
- Code -6: CRC verification failed. The data in the archive doesn’t match its integrity check, which almost always means a corrupted or incomplete download.
- Code -7: The archive header is unreadable. This can point to faulty RAM rather than a bad file, since the header is processed in memory.
- Code -11: The write operation failed. Your drive may be full, or the installer doesn’t have permission to write to the target folder.
- Code -14: A general decompression error, often linked to hardware problems like failing storage or unstable memory.
If you don’t see a specific code and just get a generic “decompression failed” message, the troubleshooting process is the same: work through the most common causes below, starting with the simplest fixes.
Corrupted or Incomplete Downloads
The single most common reason for decompression failure is a file that didn’t download correctly. If your internet connection dropped briefly during a large download, or if the server cut off the transfer early, the archive will be missing chunks of data. The CRC check catches this instantly. Re-downloading the file from scratch fixes the problem in most cases. If you’re installing a Steam game, verifying the integrity of game files through Steam’s settings will identify and replace any corrupted files automatically.
Antivirus software can also silently quarantine parts of an archive during download, leaving it incomplete. If you’re installing something you trust, temporarily disabling your antivirus before downloading and installing can prevent this interference.
Faulty RAM and Bit Flipping
Decompression is memory-intensive. Your computer loads large chunks of the archive into RAM, processes them, and writes the results to disk. If your RAM has a defect, it can silently change bits of data as they pass through. A large-scale study by Google found that memory errors fall into two categories: soft errors, which randomly corrupt bits without physical damage, and hard errors, which corrupt bits repeatedly due to a physical defect like a stuck bit. Higher temperatures increase the likelihood of these errors by raising electrical leakage inside the memory chips.
If you’re getting decompression failures across multiple different files or installers, not just one, faulty RAM is a strong suspect. Windows includes a built-in memory diagnostic tool (search “Windows Memory Diagnostic” in the Start menu) that can test your RAM for errors. Unstable RAM overclocks or XMP profiles are another frequent culprit. Resetting your memory to its default speed and timings is a quick way to rule this out.
Disk Space and Storage Problems
Decompression typically needs more free space than you’d expect. The installer needs room for both the compressed archive and the fully expanded files, plus temporary working space. If your drive fills up mid-process, the write operation fails and you get a decompression error. Check that you have significantly more free space than the stated install size, especially on the drive where your system stores temporary files (usually C:).
A failing hard drive or SSD can also produce decompression errors. If your storage device has bad sectors, writes may silently fail or corrupt data as it’s being saved. Tools like CrystalDiskInfo can show you the health status of your drive.
Permissions and Temp Folder Issues
Your operating system uses a temporary folder (typically C:\Windows\Temp) as a staging area during decompression. If your user account doesn’t have full read and write permissions to that folder, the installer can’t do its work. This is a known trigger for decompression errors in security software and other installers. Adding “Modify” permissions to the Temp folder for your user account resolves it. You can also try running the installer as an administrator, which bypasses most permission restrictions.
On Windows, User Account Control settings and restrictive group policies in work or school environments can block write access to certain directories without any visible warning. If you’re on a managed computer, this may require IT support to resolve.
Quick Troubleshooting Order
Since the most common cause is a bad download and the least common is failing hardware, work through fixes in this order:
- Re-download the file. Delete the original and grab a fresh copy. If possible, use a wired connection instead of Wi-Fi.
- Disable antivirus temporarily during download and installation.
- Run as administrator. Right-click the installer and select “Run as administrator.”
- Check free disk space. Ensure you have at least double the expected install size available on the target drive and on C:.
- Test your RAM. Run Windows Memory Diagnostic or the free tool MemTest86. If you’ve overclocked your memory, reset it to default speeds first.
- Check drive health. Use CrystalDiskInfo or a similar tool to look for signs of a failing drive.
Most people solve the problem at step one or two. If decompression errors keep appearing across different files and programs, that pattern points toward a hardware issue with your RAM or storage rather than any single bad download.