Abstract: In a device for keeping a valve controlled flow passage for supplying a reduction agent to a catalytic converter in an exhaust system of a diesel engine, wherein the reduction agent is gaseous above a certain temperature but solid below this temperature, the valve being a control valve for dosing the flow of the gaseous reduction agent and including a valve seat and a movable valve control surface, the valve surface is formed by a controllable element which can be energized selectively to be cooled or heated so that, with the valve closed and the valve surface cooled to form a condenser element gaseous reduction agent in the flow passage is solidified at the valve surface but, with the valve surface heated is readily again decomposed so as to become gaseous. As the cooling of the valve surface forms adjacent the cooled valve surface a cold trap the resolidifed reduction agent precursor formed at the valve surface where it can easily be again decomposed by the heating of the valve surface.