New baseboard heater run by thermostat connected to 3 heaters, that were original in 1971. All three heaters had stopped working at different times. Replaced the 1 that had stopped last. At the time I got 120 volts from wire. After 3 days, it stopped working. I now get 17 volts from neutral (white) wire only. Checked other two, and one gets 7 volts, and the other 43, again both just from the neutral wires, with nothing from the power (black). What am I probably looking at?