Project Zomboid "Connection Failed" and Error 4: Every Reason and Fix

Every reason Project Zomboid shows under "Connection Failed", what error 4 (a Steam P2P timeout) means, and the fix for each on a dedicated server.

Updated October 28, 2026

Project Zomboid

"Connection Failed" in Project Zomboid is the game's general join-failure screen. The useful part is the line printed under it, which is the actual reason, such as "The server failed to respond", a version mismatch or a missing mod. Read that line first, because each one has a different fix. If the reason is "Failed to establish P2P connection with server", or your log shows OnP2PSessionConnectFail: 4, that is the "error 4" people search for. In Steam's networking API, code 4 is a timeout.

We host Project Zomboid servers, and this guide works through every reason the client can show, in the order we check them on a server.

Checked 28 October 2026

Every message below is quoted from the English text files of the current public Project Zomboid build, downloaded from Steam on 28 October 2026. Build 42.21 is the stable branch (released 28 September 2026), and a 42.22 pre-release came out on 27 October 2026. That pre-release matters for the version mismatch below.

Find the Real Reason

The join screen puts the reason on the line under Connection Failed, and it stays there until you click Back. Take a screenshot before you click away. The client log in your user folder (Zomboid/console.txt on a normal install) has the same information if you missed it.

Message under "Connection Failed"	What it means	Fix
`The server failed to respond`	Nothing answered at that IP and port	Check the server is up and the address is right (step 1)
`Failed to establish P2P connection with server`	Steam could not open a connection to the server (the "error 4" case)	Step 2
`Client version (…) does not match server version (…)`	You and the server are on different builds or branches	Step 3
`Mod "…" is not installed`, `Error downloading workshop item`, `Workshop item version is different than the server's`	Your mods do not match the server's	Step 4
`Wrong server password given`	The server password is wrong	Ask the admin for the current password
`Wrong username or password given`	The account name exists on this server with a different password	Use the password you first joined with, or ask an admin to reset it
`The server is full`	Max players reached	Wait, or raise the limit
`You are banned from this server`	Your account or Steam ID is banned	Only an admin can lift it
`Your ping is too high for connection to this server.`	The server's `PingLimit` is set below your ping	Raise or disable `PingLimit` (step 5)
`Failed connection to TCP download port …`	The client could not reach the server's download port	Step 1, then your firewall
`Connection to the server was lost`	You were connected and then dropped	Server console first, then step 5

1. Is the Server Up, and Is the Address Right?

"The server failed to respond" nearly always means one of three things:

The server is still starting. On our Project Zomboid servers the console prints SERVER STARTED when it is ready to accept players. A first boot downloads the whole server and takes several minutes, and a heavily modded server also has to download every Workshop item before it starts.
The wrong port. Use the IP and port from Address in your panel. Project Zomboid's default port is 16261, but the panel assigns the port, and on a hosted server it is often a different one.
The server crashed while loading. Look in the console for a stack trace or a boot loop. A crash during load usually comes from a mod, and How to install mods for Project Zomboid covers how to isolate one.

2. "Failed to Establish P2P Connection" and Error 4

When you join through Steam, the client opens a Steam networking session to the server. If that session fails, the client logs a line like this:

znet: OnP2PSessionConnectFail: 4

The number is Steam's own error code. Steamworks documents code 4 (k_EP2PSessionErrorTimeout) as "The connection timed out because the target user didn't respond". So "error 4" is a timeout. The other values are 1 (not running the same game), 2 (does not own the app) and 3 (not connected to Steam).

Work through these in order:

Restart Steam and the game. It is the cheapest test, and it rules out a Steam client that has lost its own connection.
Check the server is a Steam server and you are a Steam client. The game warns about this itself: "This Steam client can only connect to Steam servers" and "This non-Steam client can only connect to non-Steam servers." Our Project Zomboid servers run in Steam mode, because the startup command has no -nosteam. If you added -nosteam to your launch options, remove it.
Open Steam's ports outbound. Steamworks notes that firewalls can block these sessions and says to "make sure that UDP ports 3478, 4379, and 4380 are open in an outbound direction". Work, school and some router firewalls block them.
Allow the game through your firewall or antivirus. In a 2022 Steam forum report of this exact log line, the player who solved it found Windows' built-in protection blocking the game, and whitelisting it fixed the problem.

What we have not verified

Some sites describe "error 4 followed by error 6" during Workshop downloads. We could not find "error 6" in the game's text or in Steam's P2P error list, so we cannot tell you what it is. If you see it, treat it as a mod download problem and work through step 4.

3. Version and Branch Mismatch

Client version (…) does not match server version (…) means the two sides are on different builds. Project Zomboid runs several branches at once, and Steam updates your client automatically while a server only changes build when someone updates it. This is the main cause of failed joins right after an update, and it is likely to come up now that the 42.22 pre-release is out.

Pick the same branch on both sides. Your client's branch is under Steam → Project Zomboid → Properties → Betas. On our servers the branch is the Steam Beta Branch variable on the Startup tab. Leave it blank for the default stable branch, or enter the branch name.
The branch only changes on reinstall. The variable is labelled "requires reinstall", and the install runs SteamCMD's app_update 380870 with that branch and validate. Change it, then click Reinstall Server. Back up first. A world made on a newer build should not be loaded on an older one.
After a stable patch, a reinstall brings the server up to the current build.

Project Zomboid multiplayer explains the three branches and which one to run.

4. Mod Mismatches

When you join a modded server, the client downloads the server's Workshop items before it loads the world. These are the messages you see when that goes wrong:

Mod "…" is not installed: the server requires a mod your client does not have active.
Error downloading workshop item or Error subscribing to workshop item: Steam could not fetch one of the server's items. The detail line names the Workshop item ID.
Workshop item version is different than the server's: the game explains this one itself: "Possibly the server's version is outdated, requiring the server to be restarted. If your version is outdated, try restarting Steam to get the latest version."

To fix them:

The server owner restarts the server after any mod author pushes an update, so the server picks up the new version.
Players restart Steam so their copy updates. Subscribing to the server's collection ahead of time saves a long download at the join screen.
If one item ID keeps failing, check its Workshop page. A mod that has been removed or made private cannot be downloaded, so take it out of the server's WorkshopItems and Mods lists.

If you get past this step but then see "File doesn't match the one on the server", that is a different check with its own guide: "File doesn't match the one on the server".

5. Dropped After Joining

Connection to the server was lost, or a disconnect a few seconds into the game, is a different problem from a failed join.

Server console first. If everyone dropped at once, the server stopped or restarted. Look for the crash.
PingLimit. This setting in the server's .ini kicks players whose ping goes above the value in milliseconds. 0 disables it, and that is the game's default. If a player gets "Your ping is too high for connection to this server." or keeps dropping, check the value. Project Zomboid server settings shows where the file is on our servers.
Your connection. Try wired instead of Wi-Fi, and switch off a VPN.

Common Questions

What does error 4 mean in Project Zomboid?

It is Steam's P2P error code 4, a timeout: the client could not reach the server through Steam in time. Restart Steam, make sure you are not launching with -nosteam against a Steam server, and allow outbound UDP 3478, 4379 and 4380 plus the game itself through your firewall.

Why does Project Zomboid say "Connection Failed" with no reason?

The game falls back to a bare "Connection Failed" when the connection drops before the server sends a reason. Check the server is up and you have the right IP and port, then look in console.txt for the line logged just before it.

Do I need to port forward to join a hosted server?

No. A hosted server is already reachable. Port forwarding only matters when you run the server on your own network. On a rented Project Zomboid server, the job is to use the exact IP and port from the panel.

Can a non-Steam (GOG) player join a Steam server?

No. The game itself says a Steam client can only connect to Steam servers and a non-Steam client only to non-Steam servers. A mixed group needs a server started with -nosteam, and then every player has to launch with -nosteam too.