Why is the airpot light flashing yellow

A persistent, flashing amber light indicates that any of the these potential conditions may require your attention:

• The base station has not yet been configured because it is new, or it has been reset.
• You have selected a security configuration that is not recommended.
• A new firmware update is available for the base station.
• One of several other conditions, such as a disconnected Ethernet cable or invalid IP address has been detected.

Ref:

• About the status light on AirPort base stations - Apple Support
• If your base station's light is flashing - Apple Support 

To find out why your base station's status LED is flashing amber:

1. Open AirPort Utility
2. Click on the AirPort base station
3. Look for a Status setting in the small window that appears. Note: Status only appears if there is an issue or notification.
4. Click on the small amber dot there and another window will open to explain what the "issue" might be.

Find out why the AirPort is flashing amber by........

Opening AirPort Utility on your Mac (Finder > Applications > Utilities > AirPort Utility)

Click on the AirPort to select it and a smaller window with information about the AirPort will appear.

If you see an Update button, click on it to update the firmware on the AirPort.

Otherwise, look for a message next to Status and report on the exact text that you see there.