A method at an exhaust gas cleaning system for an engine (235) in which a reducing agent is added to a passage (290) for exhaust gases from the engine (235) for cleaning the exhaust gases. The exhaust gas cleaning system includes arrangements (270) that require a certain temperature level (Tmax) in order to achieve catalytic exhaust gas cleaning. The method is to distribute and store (s430) a limited amount of reducing agent at temperature level (Tmax); and to use (s440) during a cold start of the exhaust gas cleaning system, the distributed and stored reducing agent to achieve the catalytic exhaust gas cleaning. Also a computer program product (200; 210) to implement the method. Also an arrangement for an exhaust gas cleaning system for an engine (235), and a motor vehicle (100) equipped with the arrangement.