Your HEIC photos open when you double-click them, and File Explorer still shows a wall of grey rectangles. You installed the codec. It clearly worked. So why does the folder look broken?
Because opening a photo and drawing a thumbnail are two different jobs done by two different parts of Windows. Fixing HEIC thumbnails not showing usually means clearing a cache rather than installing anything else.
Two systems, one symptom
When you double-click a photo, the Photos app loads it through the installed codec. When Explorer draws a folder, a shell extension generates a small preview and stores it in a cache database on disk.
That cache is the problem. Windows generates a thumbnail once, writes it down, and reuses it forever. If it tried before the codec existed, it recorded a blank — and it has no reason to try again.
So a working Photos app and a folder full of grey boxes is not a contradiction. It is the expected outcome of installing the codec after the files were already indexed.
Fixing the grey box instead of photo problem
Clear the cache and let Windows rebuild it. The built-in Disk Cleanup tool does this without any command line work.
- Press the Windows key and type Disk Cleanup, then open it.
- Choose your system drive.
- Tick Thumbnails in the list. Untick anything you are unsure about.
- Click OK, confirm, and wait.
- Restart Explorer, or just restart the machine.
Open the folder again. Windows now regenerates every preview from scratch, which takes a few seconds for a normal folder and longer for a library of thousands.
Worth knowing: this is entirely safe. The cache is derived data. Deleting it cannot touch a photo, and the worst case is that browsing feels sluggish while previews rebuild.
When to rebuild windows thumbnail cache manually instead
Disk Cleanup occasionally leaves the databases in place on machines with several user accounts. If that happens, delete them directly.
The files live in your user profile under AppData, Local, Microsoft, Windows, Explorer, and they are named thumbcache followed by a size number. Close Explorer first, because Windows keeps them locked while it is running.
You will see separate files for each icon size. That is why thumbnails sometimes work at one view size and fail at another — each size has its own cache, and they can go stale independently.
If blanks return immediately, the codec is the real problem
Here is the useful part of clearing the cache: it turns a vague symptom into a clear test.
On a fresh cache, Windows must generate every thumbnail from the actual files. If they come back blank straight away, nothing is generating them, and you are missing the decoder rather than fighting a cache.
That points back to the two-package problem. HEIF Image Extensions handles the container and the HEVC package decodes the picture, and having only the first is what produces black frames and empty previews together. We walked through installing the HEIF and HEVC extensions in the order that makes the diagnosis obvious.
Why the heic preview pane blank case is different
The preview pane on the right side of Explorer uses its own handler, separate from the thumbnail provider. They can fail independently, which confuses people who assume one fix covers both.
Third-party software is the usual cause. Shell extension managers, PDF suites and some backup tools register themselves as preview handlers for file types they do not fully support, taking over the pane and then failing to render.
If thumbnails work and the pane does not, that is the signature. Look at what you installed recently rather than at the codec.
When explorer shows generic icon placeholders instead
A generic icon is different from a grey box, and the difference tells you something.
A grey or black rectangle means Windows tried to render a preview and got nothing. A generic file-type icon means it did not try at all, usually because there is no content to read.
That second case is nearly always cloud storage. OneDrive and iCloud both keep files online-only by default, leaving a placeholder on disk that downloads on demand. Windows cannot preview a file whose content is not there.
Right-click the folder and choose Always keep on this device. Once the real files land, previews appear.
The photos app cannot open this file message
Sometimes one specific photo shows this while everything around it works. That is not a codec issue and no amount of cache clearing helps.
A file copied off a phone with the transfer interrupted has a perfectly valid header and missing picture data. Windows reads the header, decides it is a HEIC, tries to decode, and finds nothing there.
Before assuming the worst, test the file itself. The checker compares the sizes each internal chunk declares against the real file size, which catches a truncated transfer in a fraction of a second. If it comes back complete, your decoder is at fault. If it comes back cut short, re-copy from the phone.
Working around it without fixing Windows
If this is a machine you cannot modify, or you just need the photos today, skip the whole thing.
You can preview the photo without a codec in a browser tab, which sidesteps Explorer entirely. It will not give you thumbnails in your folders — nothing web-based can — but it shows you the images.
For a permanent fix on a locked-down machine, convert. Explorer has drawn JPG and PNG thumbnails for decades, so converted copies simply work. A lossless PNG copy instead avoids adding another generation of compression, at the cost of considerably larger files.
A short diagnostic order
| Symptom | Most likely cause |
|---|---|
| Grey boxes, photos open fine | Stale thumbnail cache |
| Grey boxes return on a cleared cache | Missing HEVC decoder |
| Generic file icons, no preview attempt | Cloud placeholder, file not downloaded |
| Thumbnails fine, preview pane blank | Third-party preview handler conflict |
| One file fails, the rest are fine | That file is damaged |
Getting your folders looking normal again
Start with Disk Cleanup and the Thumbnails box, then restart. That resolves HEIC thumbnails not showing for most people, because most people installed the codec after Windows had already cached a folder full of failures.
If the blanks come straight back on a fresh cache, you are missing the decoder rather than fighting stale data, and that is a different fix with a clear path. Either way you now know which one you are dealing with, which is more than the grey boxes were telling you.