A method for diagnosing a bypass valve in a bypass line for an intercooler in a supercharger for an internal combustion engine, including the following steps: ascertaining a value of an efficiency of the intercooler as the reference efficiency; actuating the bypass valve to open the bypass valve; ascertaining a value of a further efficiency of the intercooler as the diagnosis efficiency after actuating the bypass valve to open; and detecting a malfunction of the bypass valve when the absolute value of the deviation between the reference efficiency and the diagnosis efficiency is less than a specified threshold value.