A method for operating a fluid circuit of a motor vehicle, used for operating via a main circuit, and with a secondary circuit that is connected through a mechanical thermostatic valve to the main circuit, and an electric fluid conveying device for conveying a fluid. In a test mode, during a test period, the fluid conveying device is adjusted to a test operational speed and the actual amperage of the current required for the fluid conveying device is determined, wherein with an actual temperature of the fluid, which is lower than a switching temperature of the thermostatic value, the actual current amperage is stored as the first amperage value, and with an actual temperature, which is higher than the switching temperature, a second amperage value is stored, so that a proper function of the thermostatic valve is recognized when the second amperage is higher than the first amperage value.