<!-- 5xx server errors: code list, meaning and 509 — Uptimely -->
> Markdown version of https://getuptimely.com/errors/5xx — the page itself is the canonical copy.

Error guide

# 5xx server errors

The class meaning the server failed to fulfil a request that was, as far as it could tell, valid.

A 5xx status means the server failed to fulfil a request that appeared valid. Responsibility sits with the server side — the application, its dependencies, or the infrastructure in front of it. Unlike a 4xx, no change the client could make would have avoided it.

This is the class monitoring exists for, and every code in it should fail a check by default. It is also the class most visible to customers, because a 5xx usually means a request that should have worked did not.

The specific code narrows down which layer failed, which is most of the initial triage: 500 is the application throwing; 502 and 504 are a proxy reporting that something behind it failed or was too slow; 503 is a deliberate refusal because of capacity or maintenance; 501 is a method the server does not implement. Getting to the right layer quickly is worth more than any amount of guessing at causes.

__5xx error codes list: the registered server errors, plus 509 and Cloudflare's 52x codes__
| Code | Name                                | Defined in                                                                                 |
| ---- | ----------------------------------- | ------------------------------------------------------------------------------------------ |
| 500  | Internal Server Error               | RFC 9110                                                                                   |
| 501  | Not Implemented                     | RFC 9110                                                                                   |
| 502  | Bad Gateway                         | RFC 9110                                                                                   |
| 503  | Service Unavailable                 | RFC 9110                                                                                   |
| 504  | Gateway Timeout                     | RFC 9110                                                                                   |
| 505  | HTTP Version Not Supported          | RFC 9110                                                                                   |
| 506  | Variant Also Negotiates             | RFC 2295                                                                                   |
| 507  | Insufficient Storage                | RFC 4918 (WebDAV)                                                                          |
| 508  | Loop Detected                       | RFC 5842 (WebDAV)                                                                          |
| 509  | Bandwidth Limit Exceeded            | Not registered: sent by some web hosts when an account has used up its bandwidth allowance |
| 510  | Not Extended                        | RFC 2774, now obsolete                                                                     |
| 511  | Network Authentication Required     | RFC 6585                                                                                   |
| 520  | Web Server Returns an Unknown Error | Cloudflare, not registered                                                                 |
| 521  | Web Server Is Down                  | Cloudflare, not registered                                                                 |
| 522  | Connection Timed Out                | Cloudflare, not registered                                                                 |
| 523  | Origin Is Unreachable               | Cloudflare, not registered                                                                 |
| 524  | A Timeout Occurred                  | Cloudflare, not registered                                                                 |
| 525  | SSL Handshake Failed                | Cloudflare, not registered                                                                 |
| 526  | Invalid SSL Certificate             | Cloudflare, not registered                                                                 |

## Common causes

- An unhandled exception in application code.
- A dependency — database, cache, queue, third-party API — failing or timing out.
- Resource exhaustion: memory, file descriptors, connection pools, disk.
- A deploy that shipped a bug, or a configuration change that broke a connection.
- An upstream service that is down, unreachable or answering too slowly.

## How to diagnose it

1. Read the exact code first: it identifies the layer, and the layer determines where to look.
2. Check whether the error is in your application log or only in the proxy's — that alone splits the search in half.
3. Establish whether it affects all requests or a subset, which distinguishes a total failure from a partial one.
4. Correlate the start time against deploys, configuration changes and dependency incidents.
5. Check the health of every dependency before debugging your own code; most 5xx incidents are somebody else's outage arriving.

## Whose fault is it? The server's side

By definition the server side, though a 5xx is frequently caused by a dependency rather than by the application itself — which is still your incident even when it is not your bug.

## What monitoring sees

Uptimely's default criteria fail a check on a 5xx and record the exact code. A monitor declares an incident once the failure threshold is met — two consecutive failed checks by default — so a single blip is recorded as downtime without paging anyone, while a sustained 5xx opens an incident and runs the on-call policy.

## Related error guides

- [500 Internal Server Error](https://getuptimely.com/errors/500)
- [502 Bad Gateway](https://getuptimely.com/errors/502)
- [503 Service Unavailable](https://getuptimely.com/errors/503)
- [504 Gateway Timeout](https://getuptimely.com/errors/504)
- [501 Not Implemented](https://getuptimely.com/errors/501)
- [508 Loop Detected](https://getuptimely.com/errors/508)
- [4xx client errors](https://getuptimely.com/errors/4xx)

## Terms used on this page

- [Incident](https://getuptimely.com/glossary/incident)
- [Failure threshold](https://getuptimely.com/glossary/failure-threshold)
- [HTTP status codes](https://getuptimely.com/glossary/http-status-codes)
- [Alert](https://getuptimely.com/glossary/alert)

## Questions

### Should every 5xx trigger an alert?

Every 5xx should fail the check. Whether it alerts depends on the threshold: waiting for two consecutive failures removes single blips while still recording them as downtime, which is almost always the right default.

### Which 5xx codes point at a proxy rather than my app?

502 and 504 are generated by something in front of your application — a reverse proxy or CDN reporting that the upstream failed or was too slow. A 500 comes from the application itself.

One of the [Uptimely HTTP and network error guides](https://getuptimely.com/errors), written for the person who operates the server rather than the one refreshing the page. Something here read wrong to you? Email **support@getuptimely.com**.