Author: steveng If you purchased your heater after 2009, when your heater is plugged in to a live circuit and receiving power, the power LED on the rear of the heater should be lit. Heaters manufactured prior to 2009, included an LED that lights when the heater is actually generating heat. These heaters can be tested with freeze spray to ensure they are operating properly. Was this answer helpful ? Yes (1) / No (0) At what temperature should my heater turn on? I placed heater in the freezer or outside to test it and it is not turning on. Why?