Opened it up and saw that the inlet valve was leaking into the bottom, tripping the sensor. Mopped up the water to reset the alarm, and ordered a replacement valve. The video was extremely helpful, though I altered one step. Where the instructor disconnects the hose from the side of the washer, I realized I could easily reach in to disconnect the electrical connection for the valve, and bring it out the side of the washer rather than the back. That saved having to disconnect and reconnect the hose. Removing the dishwasher took much, much longer than in the video, due to all the things that go wrong: other plumbing fixtures in the way; sound insulation catching on the countertop; eyes getting too old to see the parts; etc.