You created a customized Health Check to ensure Google is reachable.
All of a sudden, the Health Check is failing with a similar message as below: (from Statistics > Health Checks > user.Google > Details)
Domain name: www.google.com DNS status: success Enabled Check failed DOWN IP address: 18.104.22.168 Enabled Check failed DOWN Last status: HTTP 503 - Service Unavailable Successes (total): 0 (last): Never (consecutive): 0 Failures (total): 5 (last): Tue, 31 Jan 2017 07:30:12 GMT (consecutive): 5 (external): 0 Last response time: 872 ms Average response time: 965 ms Minimum response time: 872 ms Maximum response time: 994 ms
Google is reachable but responds with an HTTP 503 response code, with a message saying that the service is Unavailable because it suspects the request comes from a robot.
When you enable the packet captures (Maintenance > Service Information > Packet Capture) and check the response from Google to the Proxy, you may see something similar to this:
In order to have a valid Google health check, you must change its frequency. The default value of 10 seconds is too fast and causes the 503 response code.
Go to Configuration > Health Checks > General > user.Google > Edit > Override the default settings > Healthy Interval > 90 seconds.