🎮 Game Error Codes What it means & how to fix it

Borderlands 3 Co-op Won't Connect? Here's the Fix

Most Borderlands 3 co-op failures come down to cross-play/SHiFT account settings, a strict NAT type, or the game being blocked by your firewall. The fastest fix: make sure both players are linked to a SHiFT account with cross-play enabled, set an Open NAT (UPnP), and allow Borderlands3.exe through the firewall. If you're on different stores, confirm cross-play is on for both. Steps below, in order.

Borderlands 3 co-op — how connections actually route

Networking model
SHiFT-backed P2P. Crossplay (Epic<->Steam) often connects when same-store (Steam<->Steam / Epic<->Epic) times out
Best fix for join timeout
Unlink the game from the SHiFT website, restart, and re-establish the crossplay link
Router-side fix
Enable UPnP (or forward ports) and disable firewall filtering for the game; older routers degrade — power-cycle 10 min
Join gotcha
If you joined via Steam username and it fails, retry selecting the host's SHiFT username instead

Last verified: September 2026

Error codes

Unable to join game / connection failed The client couldn't connect to the host's co-op session

Cause: Cross-play disabled, no SHiFT account linked, or a NAT/firewall block.

Fix:
  1. Both players link a SHiFT account (main menu > SHiFT) and enable Cross-Play in Settings.
  2. Set NAT to Open via UPnP on your router.
  3. Allow Borderlands3.exe through Windows Firewall on both PCs.
  4. If on different storefronts, confirm cross-play is enabled on both accounts.
Session not found / host unavailable Your friend's session can't be located

Cause: Different online modes, a lobby set to invite-only, or a SHiFT service hiccup.

Fix:
  1. Have the host set the game to Online and their lobby to Friends Only or Public.
  2. Join via the in-game friends list or Steam/Epic invite rather than search.
  3. Both sign out and back into SHiFT to refresh the connection.

Fixes that work for most Borderlands 3 errors

  1. Link a SHiFT account on both PCs and enable Cross-Play in Settings — it's required for most co-op joins.
  2. Set an Open NAT type by enabling UPnP on your router.
  3. Allow Borderlands3.exe through Windows Firewall and antivirus on both machines.
  4. Set the host lobby to Online and Friends Only/Public, then invite via the in-game or store friends list.
  5. Verify game files (Steam/Epic) if joins fail after an update.
  6. Use a wired connection and restart the router to clear NAT problems.
  7. Confirm both players are on the same game version after any patch.

FAQ

Why can't I join my friend in Borderlands 3?

Usually cross-play or SHiFT isn't set up, or your NAT is strict. Link SHiFT and enable Cross-Play on both accounts, set an Open NAT via UPnP, and allow the game through the firewall.

Do I need a SHiFT account for BL3 co-op?

Yes for cross-play and reliable matchmaking. Link SHiFT from the main menu on both PCs and enable Cross-Play in Settings before trying to join.

How do I fix 'unable to join game' in Borderlands 3?

Enable UPnP for an Open NAT, allow Borderlands3.exe through the firewall, set the host lobby to Friends Only/Public, and invite through the in-game friends list.

Does Borderlands 3 support cross-play?

Yes, across PC and consoles, but both players must have cross-play enabled in Settings and a linked SHiFT account for it to work.

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