Modern Field Programmable Gate Arrays (FPGAs) posses small feature sizes, and have gained popularity in mission-critical systems. However, FPGA can suffer from faults due to the small feature sizes and harsh external conditions that are faced by a mission-critical system. Therefore, the architecture of FPGA must be tested to ensure a reliable system performance. At the same time, due to the mission-critical nature of a system, the test process should be non-intrusive, i.e., applications and FPGA regions that are not being tested remain unaffected. An online test methodology is, therefore, required that not only verifies the reliability of FPGA architecture, but also does not degrade the performance of other, running FPGA applications. In this paper, we propose an online test methodology that uses hardwired network on chip as test access mechanism, and conducts test on a region-wise basis. Importantly, the proposed test methodology exhibits a non-intrusive behaviour that means it does not affect the applications and FPGA regions, which are not being tested, in terms of configuration, programming, and execution. Our test methodology posses approx. 32 times lower fault detection latency as compared to existing schemes, respectively. (c) 2012 Elsevier B.V. All rights reserved.