A method for detecting defeat devices in an engine control unit (ECU) includes storing with a key-data collection unit, a first key-data determined during an environmental test of a vehicle. The key-data collection unit stores a second key-data determined before the environmental test. Wherein, one of the first key-data is determined by the ECU modified by a characteristic only present before the environmental test and the second key-data is determined by the ECU modified by a characteristic only present during the environmental test. An environmental testing device compares the first key-data with the second key-data to detect an anomaly, wherein the anomaly indicates the presence of the defeat device in the ECU.