Most problems with a Factorio server come down to a few causes. Start with the one that matches what you see.
Players get a version mismatch error when connecting
Why: The client and server must run exactly the same game version and mods. This often happens after the stable build updates, or if the server is on experimental or a pinned version.
Fix: Update the game on the player's computer. If the server is pinned to a version, install that same version on the client, or ask support to move the server to a newer version. If Factorio Version is latest and Auto-update server is on, restarting the server brings it to the current stable build.
The server does not appear in the public server browser
Why: Public listing needs your factorio.com account details.
Fix: Enter Server Username and Server Token, save, and restart. Friends can always join with Connect to address and the Server Address from the Overview page.
The Mods tab cannot download mods
Why: Mod Portal downloads need both your factorio.com username and token.
Fix: Fill in Server Username and Server Token, or upload the mod .zip files into the mods folder instead.
RCON does not work even though RCON Enabled? is on
Why: The remote console only starts when RCON Enabled? is on and it has a port, and the port must be an extra one you add to the server. It also needs a password.
Fix: Add a second port under Ports & Network, enter it in RCON Port, set RCON Password (no spaces or quotes), and restart.
My uploaded save is ignored and a different or new world loads
Why: Either Save Name does not match the file name, or the Scenario setting is filled in. A scenario makes the server ignore Save Name.
Fix: Clear Scenario, set Save Name to the save file's name without .zip, and restart. If you uploaded a file with the same name as the old save, the old world was replaced.
Auto Save Slots does not keep several numbered autosaves
Why: Factorio saves over the same file every interval and does not keep numbered copies, whatever Auto Save Slots is set to.
Fix: Download a copy of the save from the saves folder before risky changes.
Still stuck?
Open a ticket in the category that fits and include the last 50 to 100 lines of the console. See How to open a support ticket and How to fix a server that will not start.
