Check Timeout / Connection Timeout
This is the time in seconds the monitoring agent will try to connect to the monitored target before reporting that it is unreachable. Your contacts will also be alerted when the estimated response time is bigger than the timeout value you specified. You should not set this value too low if you don't want to receive a lot of false alerts due to connection slowdown or other network related problems.
Please notice that the "Connection timed out" error in most cases doesn't mean that your server is down - it just informs you that your target is responding too slow or is unaccessible from at least two of our monitoring locations.
There are two cases of "Connection timed out" errors:
First when your server is responding properly to all our requests but the response time is higher than the "Timeout" value you've specified.
The second case is when a TCP/IP connection was not created for a specific amount of time - normally when this happens the response time is around 3 minutes (180 sec). This case is not affected by the "Timeout" value in your configuration because the TCP timeout is set at lower level and cannot be modified at run time.