I came downstairs after work and daisy_knotwise informed me that the hot water coming upstairs seemed to be distinctly anemic. So I headed down to the basement and watched the water heater's signaling light pulse out six flashes (one would be good; six, not so much). I looked at the legend on the device and it said that this indicated that the "Upper sensor failed".
Ok, Google is my friend, so I pulled out the cellphone and went digging for information, typing in the ludicrously long model number on the hot water heater. And I discovered shortly thereafter that this message seems to be endemic on this particular model of water heater. Joy.
On the other hand, someone reported that he'd managed to revive it by playing with the connectors on the sensor, since there really isn't any strong reason for the sensor to have gone bad. If only I could figure out how to get to it, which would require knowing where it was located. The tiny pictures on the cellphone weren't much help, so I retreated to the studio and fired up the computer there. On a set of instructions that someone had casually photographed, I saw where the port was and was able to go out and pull off the plug covering it. There was nothing too obvious to play with there, but I pushed it around a bit anyway.
Then I settled down on the floor and wiggled the contacts coming into the main control box.
And I tried to relight the pilot.
And failed. Several times.
But eventually, I got the pilot relit.
And as of about an hour ago, the device was still happily sending out a single pulse of light, so it appears to be working correctly.
I hope that it continues to do so, because it is going to be annoying if it doesn't...