"Disc read error" on Steam. Fix: Verify game file integrity (Right-click MW3 in Steam → Properties → Installed Files → Verify integrity).
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Multiplayer won't connect (Server offline error). Fix: MW3 official matchmaking still works, but it's laggy. Use the Plutonium launcher (a third-party client that revives dedicated servers). Note: This requires a legal Steam copy. "Disc read error" on Steam