An MP4 file will not play: what to check
Why an MP4 extension does not guarantee playback, and how to check a saved video without damaging your copy.
Check that the transfer finished
Before troubleshooting playback, open your browser’s Downloads list. A failed or unfinished transfer may leave a partial file that a player cannot open.
Compare the saved size with the expected size if the downloader provides it. A file containing only a small fraction of the expected data is unlikely to be complete.
MP4 is a container
MP4 describes a container that can hold video, audio and other information. The video and audio inside it use their own codecs.
Your device may recognise the MP4 container but lack support for one of the codecs inside. This is why two files ending in .mp4 can behave differently on the same device.
Separate video and audio problems
If the picture plays without sound, check the player’s volume, device output and any available audio-track selection. If sound plays without a picture, video-codec compatibility may be involved.
Try opening the same complete file in another trusted player already available on your device. This can help distinguish a file problem from a particular app’s limitation.
Renaming does not convert the file
Changing a filename from .webm to .mp4 does not change the underlying video or audio. It can make the file harder for apps to identify correctly.
Keep the original extension. If you need a different format for an editing project, use a suitable conversion workflow on a copy of media you are authorised to process.
Keep the original while investigating
Do not overwrite your only copy while testing repairs or conversions. Keep a separate original and check whether a fresh permitted download produces the same issue.
When contacting support, describe whether the file has no picture, no sound or an error on opening. Include the device and player name without sharing private media unnecessarily.