Model g50u4-48c-110-03 runs right now for a couple hours and shuts down. Also co detector went off upstairs. Is this a cracked heat exchanger

It is possible. There could be a crack in the exchanger that would cause overheating the plenum and the safety high limit is shutting down the heater. That same leak is allowing CO into the airstream. Don't DELAY REPAIR, CO can kill you.