My brother got a new Halo 4 game, still in the wrapper, brand new. When we launched the campaign, in co-op, it would send us back to the title screen. When he when in solo, it worked perfectly. Matchmaking also worked perfect for both controllers on. So I dunno what to do to fix this. We've already had halo 4 replaced for different reasons each time, and this disk is the best is the best result we've received, though I'd still like to know of a way to play co-op. Thanks!