What do the red and yellow lights on the Aquarium Co-Op heater mean?

When you first plug in the heater, a yellow indicator light will briefly turn on and then turn off again. The yellow light will only turn on again once the unit has run for 17,520 hours, which is 2 years of being plugged in full time. At this point, the heater will still continue to work, but the yellow light reminds you that it has reached the end of its expected life and it may be time to start thinking about replacing the unit to ensure the highest performance.

The red indicator light turns on whenever the heater is actively heating the water, and the temperature display is always on to show the current water temperature.