Yu-Gi-Oh! Master Duel's servers have been fairly reliable over the past three years. However, some players are running into issues that have them wondering if the game is down.
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In my experience, the Yu-Gi-Oh! Master Duel servers are normally fully functional outside of regularly scheduled maintenance periods, which typically occur once or twice a month. When a maintenance period is approaching, Konami will post a notification on the left side of the main menu. You can also access news about upcoming maintenance via the Maintenance tab after clicking the envelope icon at the top of the main menu.
When scheduled maintenance is in progress in Master Duel, players will be unable to open the game at all for several hours. If you attempt to open the game during these down periods, you will typically be greeted by a message in red text letting you know that maintenance is in progress. Once the maintenance is complete, you should be able to jump back into Master Duel after downloading a quick update that is usually relatively small in file size.
Outside of maintenance periods, I haven’t had many issues with servers being down. I have had issues with my internet dropping out, however, causing me to lose connectivity during ranked games. In cases like this, restarting your router and following other basic troubleshooting steps should do the trick.