The residential units have a red light indicator - if the unit is operating properly the light will remain consistent. If the unit is not operating properly, the light will flicker.
The large industrial/commercial units have red and green light indicators. The green light shows the unit has power. The red light shows the signal is propagating throughout the whole system. If the red light is dimmed, it may be that the unit has been placed within a short circuit and the signal is not propagating throughout the whole system.