The sensor is a temperature sensing head, not a heating tube.
If the temperature shows 0 degrees, it means there is ice in your water heater pipe, so this shows that the water is flowing but cannot rise because the middle of the pipe is blocked by ice. .
If it is convenient for you, you can pour hot water into the water pipe to speed up the melting of the ice in the pipe, or you can wait until the temperature rises to melt naturally. If your home is always cold in winter, pay attention to insulating water pipes.