Method for operating a coolant circuit of a vehicle air-conditioning system in an air-conditioning mode and in a heating mode realised by means of a heat pump function having: - an evaporator branch (2.1) having an evaporator and a first expansion member (6.1) assigned to the evaporator, - a coolant compressor, - an air-conditioning and heat pump branch (2.2) having an external condenser or gas cooler and a second expansion member (6.2) assigned to said external condenser or gas cooler in its function as a heat pump evaporator for the heating mode, wherein the air-conditioning and heat pump branch (2.2) is connected by way of a first shut-off member to the high-pressure outlet of the coolant compressor and by way of the second expansion member (6.2) to the evaporator branch (2.1), - a heating branch (2.3) having an internal heating condenser or heating gas cooler and a second shut-off member arranged downstream of the same, wherein the heating branch (2.3) is connected by way of a third shut-off member to the high-pressure outlet of the coolant compressor, and by way of the second shut-off member to the evaporator branch (2.1) and to the second expansion member (6.2), - a third expansion member (6.3) with which the heating branch (2.3) is connected to the air-conditioning and heat pump branch (2.2) such that, for outputting heat to the vehicle environment, coolant is relaxed by means of the third expansion member (6.3) into the outer condenser or gas cooler, and - a temperature measuring means (T, pT1) for determining the coolant temperature at the inlet of the internal heating condenser or heating gas cooler, wherein - in the heating mode the coolant temperature at the inlet of the internal heating condenser or heating gas cooler is limited to a maximum temperature value. - an evaporator branch (2.1) having an evaporator and a first expansion member (6.1) assigned to the evaporator, - a coolant compressor, - an air-conditioning and heat pump branch (2.2) having an external condenser or gas cooler and a second expansion member (6.2) assigned to said external condenser or gas cooler in its function as a heat pump evaporator for the heating mode, wherein the air-conditioning and heat pump branch (2.2) is connected by way of a first shut-off member to the high-pressure outlet of the coolant compressor and by way of the second expansion member (6.2) to the evaporator branch (2.1), - a heating branch (2.3) having an internal heating condenser or heating gas cooler and a second shut-off member arranged downstream of the same, wherein the heating branch (2.3) is connected by way of a third shut-off member to the high-pressure outlet of the coolant compressor, and by way of the second shut-off member to the evaporator branch (2.1) and to the second expansion member (6.2), - a third expansion member (6.3) with which the heating branch (2.3) is connected to the air-conditioning and heat pump branch (2.2) such that, for outputting heat to the vehicle environment, coolant is relaxed by means of the third expansion member (6.3) into the outer condenser or gas cooler, and - a temperature measuring means (T, pT1) for determining the coolant temperature at the inlet of the internal heating condenser or heating gas cooler, wherein - in the heating mode the coolant temperature at the inlet of the internal heating condenser or heating gas cooler is limited to a maximum temperature value.