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We present a low-cost concurrent test methodology for enhancing the reliability of RTL controller-datapath circuits, based on the notion of path invariance. The fundamental observation supporting the proposed methodology is that the inherent transparency behavior of RTL components, typically utilized for hierarchical off-line test, renders rich sources of invariance within a circuit. Furthermore, additional sources of invariance are obtained by examining the algorithmic interaction between the controller, and the datapath of the circuit. A judicious selection & combination of modular transparency functions, based on the algorithm implemented by the controller-datapath pair, yields a powerful set of invariant paths in a design. Compliance to the invariant behavior is checked whenever the latter is activated. Thus, such paths enable a simple, yet very efficient concurrent test capability, achieving fault security in excess of 90% while keeping the hardware overhead below 40% on complicated, difficult-to-test, sequential benchmark circuits. By exploiting fine-grained design invariance, the proposed methodology enhances circuit reliability, and contributes a low-cost concurrent test direction, applicable to general RTL circuits.