In this paper, we introduce universal fault diagnosis such that when applied to an unprogrammed FPGA, it locates a fault in any faulty programmed FPGA corresponding to the unprogrammed FPGA. If a faulty part in an FPGA can be identified prior to programming it, we can implement a required logic function on the fault-free part by isolating the faulty part. Then, we propose a universal fault diagnosis procedure that locates a faulty CLB in a look-up table FPGA. The complexity of the universal diagnosis procedure for FPGAs with block-sliced loading is independent of its array size, i.e., C-diagnosable.