Re-posted essay; the original posting was inadvertently misshelved.
You don't know what you don't know. And now I know I didn't know much about the workings of the Rinnai tankless water heater. After removing the front cover, I immediately realized it was a dense and confusing configuration of wires and parts. And there was a prominent sign warning, "Do not touch." Hence, in a reasonable amount of time (and before breaking something unrelated), when we can't hypothesize the cause of the problem, and, consequently, can't fix it, we should call for (technical) help. Many times our zeal, and intuitive thinking regarding the underlying science of the problem, are insufficient because we don't know what we don't know. That said, after preforming a preliminary inspection, which quickly revealed a lack of helpful subject matter intuition, I Googled Rinnai, searched for local authorized service providers. Apparently, stink bugs had infiltrated the exhaust system, the burner and fan assembly. Disconnecting the fan's connective rubber thingamajig revealed a defeated army of dried, pureed stink bugs. Whereupon, Painstakingly, they the remains were exhumed, which resuscitated the unit.