Have a similar problem, P0673 (2012 with 100K miles), replaced all 6 plugs, still P0673, replaced the control module (I did remove the coolant pipe) and the code went away for 3 weeks and then came back.
Now what? No code for 3 weeks means that the controller was most likely the culprit but how likely is it that a new controller (genuine BMW from getBMWparts.com) goes bad within 3 weeks?