<p><b>Abstract</b>—The optimal codes <it>C</it>(<it>n</it>, <it>t</it>) capable of detecting <it>t</it>-unidirectional errors in an <it>n</it>-bit vector were defined by Borden. Unidirectional errors have been observed in modern digital devices such as PLA's, ROM's, and compact laser disks. In this paper, a new approach to designing self-testing checkers (STCs) for Borden codes is proposed. The new STC is built of a self-testing code-disjoint translator of the Borden code into the 1-out-of-<it>z</it> code (<it>z</it>≥ 2) and—if <it>z</it> = 3 or 4—an STC for the 1-out-of-<it>z</it> code. The translator is built of two multi-output threshold circuits with <tmath>$\lfloor n/2 \rfloor$</tmath> and <tmath>$\lceil n/2 \rceil$</tmath> inputs and a merging network. Compared to existing STC's for Borden codes the new checker generally uses significantly less hardware. It also enjoys a modular and highly regular structure which makes it attractive for VLSI implementation.</p>