You guys rock, I am now able to kill zombies with my son. Any ideas how to fix the occasional 400 fx error?

Great, I got it to work by hosting online and changing max ping and to zombie mode. Played 1 game and now when I try to launch a game it says exceeded 400 fx something error and boots me. Now how do I fix that lol?

I now have 2 pc's in the same room, mine and my sons. I have 2 steam accts, 2 CoDWaW games, 2 map managers with downloaded MODS.  I log both accts in, click mods, click co-op, click host LAN and tried host online, invite my son or vice versa and get this msg. "Trying to join potential matc...