The invention relates to a method for detecting an unsealed location in a heat recovery system (5) of an internal combustion engine (1) of a motor vehicle. The heat recovery system (5) has at least one combustible working medium, a working medium circuit (6) with at least one EGR evaporator (7), a pump (8), and at least one expansion machine (9). The aim of the invention is to allow an early and reliable detection of leakages in the EGR evaporator (7) of a heat recovery system (5) in the simplest manner possible. This is achieved in that the internal combustion engine (1) is operated in the overrun mode, the oxygen concentration in the exhaust gas is ascertained and compared with a defined lower threshold, and if the lower threshold is undershot, a leakage in the EGR evaporator (7) is detected.