A check will be considered "down" if we are unable to connect to it, resolve the URL/IP address or if there is some other error.
Errors could be name server lookup failures, timeouts or SSL certificate errors (depending on whether you have selected to verify or not).
A retry is attempted when a check is found to be down to verify the state. Timeouts are capped at 20s, regardless of what is entered in your UI when 'fine tuning' the service.
A down status will not be recorded for error pages or HTTP status codes e.g. a 500 internal server error will be considered "up" because the check is responding. To be notified in this situation, you can create a content not found or HTTP status code alert check.
The "down" status affects the uptime % calculations.