A method and a device for operating an internal combustion engine having at least one mass flow line and a cooling device for cooling the mass flow in the mass flow line, as well as a bypass, having a bypass valve, that bypasses the cooling device. When the bypass valve is opened, the mass flow is conducted at least partly through the bypass. When the bypass valve is closed, the mass flow is conducted through the cooling device. Downstream from the cooling device and from the bypass in the mass flow line, a temperature of the mass flow in the mass flow line is determined. In at least one operating state of the internal combustion engine, a first temporal temperature gradient is determined with closed bypass valve. In the at least one operating state of the internal combustion engine, a second temporal temperature gradient is determined with closed position of the bypass valve. An error is recognized as a function of a deviation between the first temporal temperature gradient and the second temporal temperature gradient.