In Advanced Heavy Water Reactor, a set of 168 Self-Powered Neutron Detectors (SPNDs) distributed uniformly at 6 axial locations in 28 vertical In-core Detector Housing (ICDH) units each, has been proposed to be used for monitoring and control of neutron flux distribution in the reactor core. These 168 SPNDs arrived at by applying the K-means clustering approach on the simulated in-core detector measurements, proved to be effective in flux reconstruction during normal reactor operational scenarios such as the spatial flux variations caused by movement of regulating rods, xenon spatial oscillations, and on-power refuelling. However, the accuracy of flux mapping may be affected in case of the unavailability of one or multiple SPNDs. Hence the effect of the unavailability of SPNDs on accuracy characteristics of flux mapping needs to be studied. Motivated by this, the performance of the flux mapping system in case of unavailability of all 6 SPNDs of an ICDH unit under various nominal reactor operational scenarios has been studied. It is evident from the analysis results presented in the paper that though the RMS error is poorer compared to the case when all SPNDs are available, the overall RMS error is better than 7%. The studies also brought out that the ICDH units located in the outer region of the core have more bearing in the flux reconstruction.