the invention relates to a dosing pump unit for mixing a liquid reducing agent with an exhaust gas flow by means of a dosing pump which is connected , on the input side thereof , to a connection connectable to a reducing agent tank and , on the output side thereof , to a pressure pipe and a pipe for returning flow back to said reducing agent tank , wherein a non - return valve which is prestressed against a hydraulic pressure produced by the dosing pump is arranged in the pressure line , the return pipe is connected to the pressure pipe upstream of the non - return valve in the reducing agent flow direction and a first stop valve for selectively closing the return pipe is placed therein .