Use these troubleshooting steps to fix Halo 3 campaign co-op issues.

Diving into Halo 3’s campaign with an online friend (or several) sometimes simply doesn’t…work. You’ll send out the invites and group up, pick and load a mission, and then the game abruptly hangs before kicking you to the main screen. Or you’ll get to play for a short while, and then everything freezes and boots you.

Either way, you get torn apart and can even lose your progress, both of which are irritating as hell. And this experience isn’t isolated to Halo 3—it can happen with other Microsoft games that rely on Xbox Live servers for multiplayer too, be it the rest of the Master Chief Collection or Forza.
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https://www.pcworld.com/article/3568681/master-chief-collection-campaign-co-op-not-working-heres-how-to-fix-it.html