Which HTTP status codes mean a website is down?

5xx server errors (500, 502, 503, 504) always mean an outage. 4xx errors (403, 404, 429) usually don't — the server answered but rejected the request. Connection timeouts and DNS failures also count as outages.

• 500 Internal Server Error — application crashed. Real outage.
• 502 Bad Gateway — upstream server unreachable. Real outage.
• 503 Service Unavailable — server intentionally offline or overloaded. Real outage.
• 504 Gateway Timeout — upstream took too long. Real outage.
• 429 Too Many Requests — rate-limit, not an outage; investigate your check frequency.