503 Service Unavailable
The server is up but is refusing to handle requests for the moment: maintenance mode, too many requests, or no healthy backend behind a load balancer.
Where it failed: the application
What visitors see
A "503 Service Unavailable" or "temporarily unavailable" page, sometimes with a Retry-After time.
Usual causes
- Maintenance mode left switched on (WordPress leaves a .maintenance file if an update stops halfway).
- The server is overloaded: every worker is busy, or a rate limit is hit.
- Every backend failed its health check, so the load balancer has nowhere to send traffic.
- A hosting account over its resource limits (entry processes, CPU) is paused by the host.
How to tell which one it is
- A Retry-After header points at deliberate maintenance or rate limiting.
- A load balancer's 503 comes with its own page and is logged as "no healthy targets" (or similar).
What to do
- Turn maintenance mode off (WordPress: delete the .maintenance file in the site root).
- Check the load balancer's target health and why the health check fails (wrong path, slow response).
- Find what is using the workers (a traffic spike, a slow query) and add capacity or caching.
- Ask the host whether the account hit a resource limit.
Check the site now
The Down Checker names the layer that failed (DNS, connection, TLS or the server) and decodes the error page it gets.
Free, no sign-up. Scan only sites you own or may test. 60 scans an hour per address.