Embedded system face a serious threat from physical attacks when applied in critical applications. Therefore, modern systems have several integrated sensors to detect potential threats. In this paper, we put forward a new issue where these sensors can open other security loopholes. We demonstrate that sensors, which are deployed to prevent faults, can be exploited to insert effective and almost zero-overhead hardware Trojans. Two case studies are presented on Xilinx Virtex-5 FPGA. The first case study exploits the in-build temperature sensor of Virtex-5 system monitors while the other exploits a user deployed sensor. Both the sensor can be used to trigger a powerful Trojan with minimal and at times zero overhead.