Method for operating a refrigeration system for a motor vehicle, wherein the refrigeration system comprises: a primary line and a secondary line; a refrigerant compressor which can be connected or is connected to the primary line and the secondary line; a directly or indirectly acting external heat exchanger which is arranged in the primary line; an evaporator which is arranged in the primary line; at least one further heat exchanger constituting a heat source, in particular a heating register, which is arranged in the secondary line; a primary line valve which is arranged between the refrigerant compressor and the external heat exchanger; a secondary line valve which is arranged between the refrigerant compressor and the further heat exchanger, in particular the heating register, which constitutes a heat source; at least one valve apparatus (A2, A5) which is arranged between a high-pressure-side section (15, 15a) of the primary line or the secondary line and a low-pressure-side line section which is located upstream of the refrigerant compressor, characterized in that the method comprises the following steps: setting an operating mode of the refrigeration system with an active primary line and an inactive secondary line or with an active secondary line and an inactive primary line; detecting the pressure (p_inak) in the inactive line (14, 16); activating an extraction of refrigerant from the inactive line (14, 16) and into the active line (14, 16) by lowering the pressure (pA) in the active line (14, 16) to a value below the pressure (p_inak) in the inactive line (14, 16) and by opening the valve apparatus (A2, A5) in question. a refrigerant compressor which can be connected or is connected to the primary line and the secondary line; a directly or indirectly acting external heat exchanger which is arranged in the primary line; an evaporator which is arranged in the primary line; at least one further heat exchanger constituting a heat source, in particular a heating register, which is arranged in the secondary line; a primary line valve which is arranged between the refrigerant compressor and the external heat exchanger; a secondary line valve which is arranged between the refrigerant compressor and the further heat exchanger, in particular the heating register, which constitutes a heat source; at least one valve apparatus (A2, A5) which is arranged between a high-pressure-side section (15, 15a) of the primary line or the secondary line and a low-pressure-side line section which is located upstream of the refrigerant compressor, characterized in that the method comprises the following steps: setting an operating mode of the refrigeration system with an active primary line and an inactive secondary line or with an active secondary line and an inactive primary line; detecting the pressure (p_inak) in the inactive line (14, 16); activating an extraction of refrigerant from the inactive line (14, 16) and into the active line (14, 16) by lowering the pressure (pA) in the active line (14, 16) to a value below the pressure (p_inak) in the inactive line (14, 16) and by opening the valve apparatus (A2, A5) in question. setting an operating mode of the refrigeration system with an active primary line and an inactive secondary line or with an active secondary line and an inactive primary line; detecting the pressure (p_inak) in the inactive line (14, 16); activating an extraction of refrigerant from the inactive line (14, 16) and into the active line (14, 16) by lowering the pressure (pA) in the active line (14, 16) to a value below the pressure (p_inak) in the inactive line (14, 16) and by opening the valve apparatus (A2, A5) in question.