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I received one of these used. I set the temp and put it in a bucket of water that was well below the temp setting. The light did not come on. Do these things need time to warm up?
If the heater was working properly, the light should have come on instantly. Did you try adjusting the thermostat knob up and down several times ? Possibilities are a busted thermostat/contact, or a burned out heating element - neither is repairable without ruining the watertight seal.
I took it out of the water and held the heater part in one had and played with the thermostat with the other hand. The light never came on and it never got hot.