This paper addresses the issue of characterizing the respective impact of fault injection techniques. Three physical techniques and one software-implemented technique that have been used to assess the fault tolerance features of the MARS fault-tolerant distributed real-time system are compared and analyzed. After a short summary of the fault tolerance features of the MARS architecture and especially of the error detection mechanisms that were used to compare the erroneous behaviors induced by the fault injection techniques considered, we describe the common distributed testbed and test scenario implemented to perform a coherent set of fault injection campaigns. The main features of the four fault injection techniques considered are then briefly described and the results obtained are finally presented and discussed. Emphasis is put on the analysis of the specific impact and merit of each injection technique.