Factorio Server Troubleshooting

Factorio servers are genuinely stable compared to most game servers I deal with. When something does go wrong, though, the error messages can be cryptic. Here's what I've actually seen people run into and how to fix it.

Can't Connect to the Server

This is the most common support request and it's almost never the server's fault.

First, verify the basics. Your server IP and port are in the Service Settings section of your control panel at control.low.ms. The game port is UDP 34197 by default. Make sure you're connecting to the right address – copy-paste it rather than typing it.

If you're getting timeouts, the issue is usually between you and the server rather than the server itself. Factorio uses UDP, and some routers handle UDP poorly. Check if the server shows as online in your control panel. If it's running and you still can't connect, try these:

Restart your router (yes, really – NAT table issues are a thing)
Temporarily disable your local firewall to test
Try connecting from a mobile hotspot to rule out your ISP

If require_user_verification is true in server-settings.json and you're getting authentication errors, make sure your Factorio account is properly linked. You can check this at factorio.com. The server validates every connection against their auth servers.

Password issues are simpler – double-check you're entering the game_password from server-settings.json, not any other password. It's case-sensitive.

Desync Errors

Desyncs are probably the most misunderstood issue in Factorio multiplayer. Here's what's actually happening.

Factorio uses deterministic lockstep networking. That means the server and every client are all running the full game simulation independently. The server doesn't send world state – it only sends player inputs, and every machine computes the world from those inputs. This is why Factorio multiplayer scales so well, but it also means every machine must arrive at exactly the same game state on every single tick.

A desync happens when one machine's simulation diverges from the others. This is not caused by network latency or packet loss. I want to be very clear about that because it's the most common misconception. You could play on a 500ms connection and never desync. Latency causes rubber-banding and input delay, not desyncs.

Actual desync causes:

Factorio bugs – rare in the base game post-2.0, but they happen. Update to the latest version.
Mod bugs – much more common. Mods that don't handle determinism correctly can cause desyncs. If you're desyncing and running mods, try disabling them one by one to find the culprit.
Bad RAM – this is the one nobody wants to hear. If your local machine has failing memory, the simulation will compute wrong results occasionally. Run a memory test (memtest86) if you're desyncing with no mods on the latest version. On the server side, our hardware is ECC memory, so this is almost always a client-side issue.

After several desyncs in a session, Factorio automatically disconnects the player. Reconnecting lets you try again.

When a desync occurs, Factorio writes a desync report to the logs. If you're reporting it as a bug, that report is essential – it contains the exact tick where simulations diverged.

UPS Drops and Performance

UPS – Updates Per Second – is Factorio's heartbeat. The game targets 60 UPS, and when the server can't keep up, the entire game slows down. Not just framerate – actual game speed. Everything runs in slow motion.

Here's the critical thing to understand: Factorio is primarily single-threaded. Having more CPU cores doesn't help. What matters is single-core clock speed and – this is a big one – CPU cache size. There's a reason the Factorio community specifically recommends AMD X3D processors (5800X3D, 7800X3D). The massive L3 cache on those chips makes a measurable difference because Factorio's simulation accesses huge amounts of data semi-randomly.

Performance killers, roughly in order of impact:

Active entities – inserters, assemblers, furnaces. Each one is simulated every tick. A mega-base with 50,000 inserters is going to hurt.
Logistics bots – each bot is a pathfinding entity. A few thousand are fine. Tens of thousands start to add up.
Belts – optimised heavily in recent versions, but still contribute at scale.
Fluid calculations – pipes are surprisingly expensive. Keep fluid networks simple.
Biters – large biter populations, especially during attacks, spike CPU usage. If you don't care about combat, turn them off for better performance.

Player count itself barely affects performance thanks to the lockstep architecture. 2 players or 20 – the server does the same simulation work. What matters is factory complexity.

The /toggle-heavy-mode command adds extra validation per tick, which helps debug issues but tanks performance. Make sure it's not accidentally enabled.

You can check your server's UPS through the Current Activity & Stats section in the control panel. If it's consistently below 60, you need to optimise your factory, not your server. There are excellent guides on the Factorio subreddit and forums about UPS-optimised builds.

Memory Usage

Factorio starts modest – around 1GB for a fresh vanilla world. It scales from there based on:

Map size explored
Number of entities built
Mods installed (some modpacks add enormous amounts of content)
Save file size

A typical mid-game factory with a handful of mods sits around 2-4GB. Heavy modpacks like Space Exploration or SeaBlock with a late-game base can push 6-8GB or beyond. Our plans start at 10GB which handles the vast majority of cases, with upgrades to 30GB for the real monsters.

If your server is running out of memory, you'll see crashes in the logs – often during saves, when a big allocation fails and the process bails out. Check your memory usage in the control panel and upgrade if you're consistently above 80%.

Mod Mismatches

"Mods are not the same" – every Factorio multiplayer admin's favourite error.

Factorio enforces exact mod version matching between server and all clients. The checksums must match. There's no "close enough."

Troubleshooting steps:

Check which mods and versions the server is running – look at mod-list.json in the mods/ directory through your File Manager
Make sure connecting players have the same list. Factorio's multiplayer screen has a "sync mods with server" option that downloads the correct versions automatically – use it
If a mod updated recently and half your group auto-updated while the server didn't, either update the server (Steam Update in the panel) or have players roll back their local mods
Space Age DLC is treated as a mod. If it's enabled on the server, all connecting players need to own it

For mod-settings.dat – if mod settings differ between server and client, some mods won't care but others will refuse to connect. When in doubt, configure mods locally, upload the dat file to the server, and have all players sync.

Save File Issues

Save corruption

Genuine save corruption is rare but devastating. Causes include:

Server running out of memory during a save operation
Disk full (check via File Manager)
Bad RAM causing bit-flips in the save data

This is why autosave rotation exists. If your latest save is corrupted, grab a previous autosave from the saves/ directory. I recommend bumping autosave_slots to at least 10 in server-settings.json for exactly this reason. Cloud Backup in the control panel is another safety net – use it before making big changes.

Creating a new save

If you need to start over, remember that you must create a save before the server will start. Use the Web Console:

/bin/x64/factorio --create ./saves/new-world.zip --map-gen-settings ./data/map-gen-settings.json

Then update your startup configuration to point at the new save file through the Commandline Manager.

Joining Takes Forever

When a player joins a Factorio server, the server sends them the current save file, and their client has to fast-forward through the simulation to catch up to the current tick. On a large save, this can take a while.

The bottleneck is usually the joining player's CPU – they're essentially replaying the game state at maximum speed. There's nothing you can do server-side to speed this up beyond keeping save files reasonable.

max_upload_slots in server-settings.json controls how many players can download the map simultaneously. If several people try to join at once on a large save, it can bottleneck. Default is 5, which is fine for most cases.

If joining consistently takes more than a few minutes, your save might just be very large. That's the nature of a 200-hour mega-base.

Space Age DLC Compatibility

Since the Space Age DLC (and the accompanying 2.0 update) launched in October 2024, there are a few things to keep in mind:

The DLC is treated as a collection of mods internally (space-age, quality, elevated-rails). Enable or disable them in mod-list.json or the Mod Manager.
All players need to own the DLC if it's enabled on the server.
Base game 2.0 works fine without the DLC – you still get the engine improvements, just not the DLC content.
Some older mods haven't been updated for 2.0. Check mod compatibility before updating the server.

If something's not covered here and you're stuck, check the Log Viewer in your control panel. Factorio's logs are actually quite informative – most errors include enough context to point you in the right direction. And if you're really stuck, our support team can take a look.