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Accumulation of dormant faults is a potential threat in a fault tolerant system, especially because most often fault tolerance is based on the single-fault assumption. We investigate this threat by the example of an automotive steer-by-wire application based on the Time-Triggered Architecture (TTA). By means of a Markov model we illustrate that the effect of fault dormancy can degrade the MTTF of a system by several orders of magnitude. We study potential remedies, of which transparent online testing proves to be the most powerful one, while taking a hot spare offline temporarily to test it provides a more feasible solution, though with tight constraints regarding the test duration.