A control device for a blood treatment machine comprising an extracorporeal blood flow circuit with first and second access devices (2', 2") for connection to upstream and downstream portions, respectively, of a vascular access of a patient and having a blood pump operable to generate a flow of blood in the extracorporeal blood flow circuit from one of the first and second access devices (2', 2") through a blood compartment of a dialyzer and to another of the first and second access devices (2', 2"), and a treatment fluid flow circuit configured to generate a flow of treatment fluid through a treatment fluid compartment of the dialyzer , said treatment fluid compartment being separated from the blood compartment by a semi-permeable membrane , said control device being configured to, during a connection test: cause the blood treatment machine to switch between a first operating state in which the blood pump is operated in a default direction to pump the blood from the first access device (2'; 2") through the blood compartment of the dialyzer to the second access device (2"; 2'), and a second operating state in which the blood pump is operated in a reverse direction to pump the fluid from the second access device (2"; 2') through the blood compartment of the dialyzer to the first access device (2'; 2"), acquire an output signal (S1, S2) of at least one sensor in the blood treatment machine , compute, based on the output signal (S1, S2), an efficiency change parameter that represents a change in in-vivo clearance of the blood treatment machine during the switch of the blood treatment machine between the first and second operating states, and wherein said control device is further configured to evaluate the efficiency change parameter to determine if the first or second operating state involves a dual fault condition comprising both a co-current dialyzer configuration, in which the flow of blood through the blood compartment and the flow of treatment fluid through the treatment fluid compartment are in a common direction along the semi-permeable membrane , and a reversed access device configuration, in which the first and second access devices (2', 2") are connected to the downstream and upstream portions, respectively, of the vascular access . cause the blood treatment machine to switch between a first operating state in which the blood pump is operated in a default direction to pump the blood from the first access device (2'; 2") through the blood compartment of the dialyzer to the second access device (2"; 2'), and a second operating state in which the blood pump is operated in a reverse direction to pump the fluid from the second access device (2"; 2') through the blood compartment of the dialyzer to the first access device (2'; 2"), acquire an output signal (S1, S2) of at least one sensor in the blood treatment machine , compute, based on the output signal (S1, S2), an efficiency change parameter that represents a change in in-vivo clearance of the blood treatment machine during the switch of the blood treatment machine between the first and second operating states, and wherein said control device is further configured to evaluate the efficiency change parameter to determine if the first or second operating state involves a dual fault condition comprising both a co-current dialyzer configuration, in which the flow of blood through the blood compartment and the flow of treatment fluid through the treatment fluid compartment are in a common direction along the semi-permeable membrane , and a reversed access device configuration, in which the first and second access devices (2', 2") are connected to the downstream and upstream portions, respectively, of the vascular access . <EOS>