Device for clearing a supply pipe of a reducing agent that has previously gone from a first phase to a second phase then back again

A device for keeping a valve controlled flow passage supplying a reduction agent to a catalytic converter in an exhaust system of a diesel internal combustion engine, said reduction agent being gaseous above a certain temperature, but solid below said certain temperature, said valve (12) being a control valve for dosing the flow of the gaseous reduction agent to the catalytic converter, said control valve including a valve seat (19) and a movable valve surface (16) which abuts the valve seat when the valve is closed, said valve surface (16) being formed by a controllable element which, when energized is selectively cooled or heated so that, upon energization of the controllable element with the valve closed and said valve surface being cooled, a cold trap structure is formed in the flow passage adjacent the valve surface (16) for solidifying gaseous reduction agent present in the flow passage on the cold valve surface and, upon subsequent energization of the controllable element, the solidified reduction agent is again converted to a gaseous phase, and a control unit for selectively controlling the energization of the controllable element for cooling or heating the controllable element.