Toward the end of 1941, she was transferred to the North Sea, and went into drydock for her last major modification. This consisted of the installation of a FuMO 21 radar set on the forward command center roof. In July 1942, "Köln" departed Germany to join the growing naval presence in Norway, though she saw no major action there. On 13 September, she and the heavy cruisers "Admiral Scheer" and and two destroyers attempted to attack Convoy PQ 18. While en route from Narvik to Altenfjord, the flotilla was attacked by the British submarine , but the torpedoes passed behind the German ships. The convoy was instead attacked by U-boats and long-range bombers, which sank thirteen freighters. She returned to Germany in January 1943, where she was decommissioned in Kiel on 17 February. She was sent to drydock in early 1944 for an overhaul to prepare her to return to combat duty; this was completed by 1 July. The cruiser served briefly as a training ship before escorting German merchant vessels in Norway. While en route from Kristiansand on 7 July, the ship laid a defensive minefield in the Skagerrak. She and three destroyers laid another minefield on 14–15 July, before steaming to Trondheim.