Abstract: A diagnostic relay has a casing mounted on a relay base and enclosing components of the relay. The casing includes a top wall having a plurality of apertures. The apertures are aligned with contacts inside the casing. The contacts are integral with or electrically connected to relay prongs traversing the base. A diagnostician replaces a circuit relay with the diagnostic relay, then inserts probes of a circuit tester through selected apertures into engagement with the contacts to determine the location of faults in the circuit. In one embodiment, the casing is clear or transparent so the diagnostician may observe whether moving components of the diagnostic relay are operating properly.