Various embodiments of the present disclosure are directed to methods and systems for determining at least one actual value of a controlled variable of a conditioning unit for a reactant of a fuel cell. In one example embodiment, the method steps for determining the at least one actual value of a controlled variable includes: measuring a measured value of the actual value of the at least one controlled variable, calculating a model value of the at least one controlled variable using a model of the conditioning unit, calculating a model value of the actual value of the at least one controlled variable using a sensor model, calculating a correction value for the at least one controlled variable, and calculating the actual value of the at least one controlled variable as the sum of the correction value and of the model value of the at least one controlled variable.