A method is provided for injecting faults into an operational system containing software and hardware components to be tested. A fault injection routine is stored in a memory location of the system. An interrupt service routine, preferably operational during normal operation of the system is provided with a pointer to the fault injection routine. A fault injection routine is executed from within the interrupt service routine. In one embodiment, the interrupt service routine provides a clock or timing function. In another embodiment, a number of fault injection subroutines are accessible by the fault injection routine. A value is passed to the fault injection routine indicating the particular fault injection subroutine to be employed.