We propose a technique for online built-in self-test of Field Programmable Gate Arrays (FPGAs). The goal of this system is to detect deviations from the intended functionality of an FPGA without using special-purpose hardware, hardware external to the device, and without interrupting system operation. A system that solves these problems would be useful for mission-critical applications with resource constraints. We present here a fault detection system which solves these problems through an online fault scanning methodology. Resources internal to the device are configured to test for faults. Testing scans across an FPGA, checking a section at a time. The viability and effectiveness of such a system is supported through simulation of the system on a model FPGA.