A method for operating a magneto-inductive flow measuring system, wherein the magneto-inductive flow measuring system has a measuring tube, wherein measuring tube has a lumen for the conveying an at least slightly electrically conductive, measured substance, and wherein the magneto-inductive flow measuring system has a first and a second electrode electrically interacting with the measured substance. In a first diagnostic operation, an input voltage is applied between the first and second electrodes, wherein the input voltage produces an electrical current essentially within the lumen, and wherein the flow measuring system additionally has a third and a fourth electrode electrically interacting with the measured substance, wherein an output voltage falling between the third and fourth electrodes essentially due to the electrical current is determined, and wherein, by means of the output voltage, a state of the flow measuring system and/or a chemical and/or physical property of the measured substance is determined.