The heater is outdoors, uncovered. It's been dry for over a week. I didn't turn it on when the pool was opened, but of course when I needed it it didn't work!
I did clean it. I also cleaned the various connectors, some of which tend to oxidize over the winter. No luck. Pressure switch was replaced two years ago, thermocouple a few years ago. Would faulty high limit switch cause this? It seems to be the only part that hasn't been replaced.