The safety-level DCS in nuclear power plants ensures the determinacy of system behavior after signal failure by detecting failed signals and setting default values. In view of the problem that the effect of default values is difficult to meet expectations due to the incomplete diagnostic coverage of the signal acquisition path in DCS, the diagnostic mechanisms and default value implementation schemes for signals with different transmission methods and types are studied. By identifying the failure modes of signal source, signal transmission link and DCS acquisition channel, the fault detection methods for failed signals are studied. At the same time, considering the diagnostic coverage and engineering cost, a scheme of fault detection mechanisms integrating different diagnostic measures and default value function implementation is proposed for different signals. Within the scope of engineering feasibility, this scheme can maximize the improvement of the fault detection mechanism after signal failure, ensuring that DCS signals can be smoothly switched to the preset default values under the expected failure modes. Meanwhile, this scheme can provide reference for the subsequent scheme design of fault detection and default value function implementation for DCS signals in nuclear power plants.