Linksys Extender has three LED lights:

1. The first one stands for power;
2. The second one stands for powerline connection;
3. And last third one stands for Ethernet connection.

Briefly Explanation:
Power LED:
Power led should be on all the time. If it is off, then change a socket to have a try. If still can’t power on, after this arrange to exchange a new one.
When it is in the pairing process or in power-save mode, the Power LED will flash. After the process is over, the Power LED will back to steady on.
Powerline LED:
When you have successfully made the powerline connection, the Powerline LED will be on. It will be off when it is in Power-save mode.
Ethernet LED:
In Ethernet Led any device when connected to the Ethernet port and be powered on, the Ethernet LED will be on. It will flash with data transfer. Only when the LED is off for more than 5 minutes, it will be in power-save mode.
Wi-Fi LED:
Wi-Fi LED is on or flashing quickly means the wireless connection is enabled. In the Wi-Fi cloning procedure, the light will flash slowly.  Press down and hold the Wi-Fi button for at least 5 seconds to turn the wireless function on or off. The wireless function is on by default. You can turn it off at night and then on in the morning by pressing this button.