Since it seemed more likely than not that the outdoor sensor was the culprit, I located and ordered a replacement sensor. I installed batteries in the new sensor first, then in the indoor unit. After about ten seconds, the indoor unit started to respond. Moved the sensor to it's usual outdoor location, and all is back to normal.

Thanks to all who responded!

Interesting footnote: The online shop I bought the sensor from is for wine enthusiasts. They sell the weather station to folks who want to monitor the temperature and humidity of their wine cellar!

Art Harris