Microcontrollers in dyad are used for increased reliability and fault tolerance. Depending on two types of software the behaviour of such system could be for increased persistency or for increased integrity. Redundancy is incorporated in software in form of time redundancy, data multiplication, retry etc. Microcontrollers in dyad are loosely coupled with task synchronisation. The evaluation of used fault tolerant methods is done by experiments using the fault injection method. The fault injection system is based on a personal computer, which controls the execution of the experiment and collects the results. The activity of the bus signals is used in calculation of the probability of the fault occurrence. The fault recovery coverage is evaluated using the registered number of faults and the probability of the fault occurrence.