OD relay: The small internal logic PCB stands vertically on the connector base, connected by what looks like the legs on a common IC chip. Vibration can break one or more of those legs loose from the thru-hole. With a good light and magnification, wiggle the PCB and watch the legs at their connections for what looks like a cold solder joint. I was able to resolder and bring mine back to life, still going strong several years later.
