"odourless" smoke is probably just steam? Fairly normal in colder weather and often worse on cars that dont get decent long journeys.
The error code would suggest that your cat has become worn out and is no longer effective at removing NOx from the exhaust gas. Theres also some suggestion online that you can "regenerate" the cat manually. EG here:
http://forum.bmw5.co.uk/topic/114925...nverter-aging/
It could, potentially, also be a faulty NOx sensor.