Diagnostic method for a dialysis fluid preparation device (1, 101), the latter comprising: a water supply channel (2, 102) for supplying water, an A-fluid channel (3, 103) for supplying an A-fluid containing sodium chloride as a main ingredient, together with at least one other ingredient, the A-fluid channel (3, 103) being connected to the water supply channel (2, 102), a B-fluid channel (4, 104) for supplying a B-fluid consisting of an aqueous sodium bicarbonate solution, with the B-fluid channel (4, 104) being connected to the water supply channel (2, 102), and concentration measuring means (20, 23, 27, 120, 123, 127), provided with a detection unit (20a, 23a, 27a, 120a, 123a, 127a) downstream of the connection positions of the A-fluid channel (3, 103) on the water supply channel (2, 102), the diagnostic method comprising: mixing the water, the A-fluid and the B-fluid, each in predetermined proportions, in order to prepare a dialysis fluid, and measuring a concentration of the prepared dialysis fluid using the concentration measuring means (20, 23, 27, 120, 123, 127), characterized by, prior to the preparation of the dialysis fluid, supplying the A-fluid from the A-fluid channel (3, 103) to the water supply channel (2, 102) such that it is only A-fluid flowing in the water supply channel (2, 102), and diagnosing a measurement accuracy of the concentration measuring means (20, 23, 27, 120, 123, 127) by measuring an electrical conductivity of the A-fluid, be it undiluted or diluted, using the concentration measuring means (20, 23, 27, 120, 123, 127) and subsequently comparing the measured electrical conductivity of the A-fluid or the undiluted A-fluid with a value set in advance. a water supply channel (2, 102) for supplying water, an A-fluid channel (3, 103) for supplying an A-fluid containing sodium chloride as a main ingredient, together with at least one other ingredient, the A-fluid channel (3, 103) being connected to the water supply channel (2, 102), a B-fluid channel (4, 104) for supplying a B-fluid consisting of an aqueous sodium bicarbonate solution, with the B-fluid channel (4, 104) being connected to the water supply channel (2, 102), and concentration measuring means (20, 23, 27, 120, 123, 127), provided with a detection unit (20a, 23a, 27a, 120a, 123a, 127a) downstream of the connection positions of the A-fluid channel (3, 103) on the water supply channel (2, 102), the diagnostic method comprising: mixing the water, the A-fluid and the B-fluid, each in predetermined proportions, in order to prepare a dialysis fluid, and measuring a concentration of the prepared dialysis fluid using the concentration measuring means (20, 23, 27, 120, 123, 127), characterized by, prior to the preparation of the dialysis fluid, supplying the A-fluid from the A-fluid channel (3, 103) to the water supply channel (2, 102) such that it is only A-fluid flowing in the water supply channel (2, 102), and diagnosing a measurement accuracy of the concentration measuring means (20, 23, 27, 120, 123, 127) by measuring an electrical conductivity of the A-fluid, be it undiluted or diluted, using the concentration measuring means (20, 23, 27, 120, 123, 127) and subsequently comparing the measured electrical conductivity of the A-fluid or the undiluted A-fluid with a value set in advance. mixing the water, the A-fluid and the B-fluid, each in predetermined proportions, in order to prepare a dialysis fluid, and measuring a concentration of the prepared dialysis fluid using the concentration measuring means (20, 23, 27, 120, 123, 127), characterized by, prior to the preparation of the dialysis fluid, supplying the A-fluid from the A-fluid channel (3, 103) to the water supply channel (2, 102) such that it is only A-fluid flowing in the water supply channel (2, 102), and diagnosing a measurement accuracy of the concentration measuring means (20, 23, 27, 120, 123, 127) by measuring an electrical conductivity of the A-fluid, be it undiluted or diluted, using the concentration measuring means (20, 23, 27, 120, 123, 127) and subsequently comparing the measured electrical conductivity of the A-fluid or the undiluted A-fluid with a value set in advance.