I've gotten this code before when my exhaust manifold was leaking where it bolted to the engine. I replaced the gasket and tightened the manifold back on and haven't seen it since. Could also be a cracked exhaust manifold. It could be a totally different problem, but that's my experience with this CEL!
It usually is a vacuum leak but can be any leak between the maf and primary O2 sensor. Cracked exhaust manifold, torn WGA diaphragm, cracked vacuum lines, broken plastic IC piping are the most likely causes.
I have this code as well... I replace all my vac lines to silicon hoping it would fix it but that wasn't the problem. I notice today I can hear a hiss around the intake mani and was wondering could it possibly be my intake mani gasket? I don't want to go around replacing parts blindly.