# Error codes

Ordnance Survey uses standard HTTP response codes to provide key information on the outcome of API requests. These codes serve as a straightforward means of communication, indicating the success or failure of each API request between your application and our APIs.

When a request is successful, you'll encounter codes within the `2xx` range. If the information in the request is incorrect (for example, missing query parameters), you'll receive codes within the `4xx` range, and `5xx` codes indicate an error with the OS APIs.

The following table details the HTTP response codes and their meaning within the context of OS APIs:

<table data-full-width="false"><thead><tr><th width="95">Code</th><th width="184">Official description</th><th>Explanation in context of the API</th></tr></thead><tbody><tr><td><code>200</code></td><td>OK</td><td>Request has been successful.</td></tr><tr><td><code>304</code></td><td>Not Modified</td><td>In response to a conditional <code>GET</code> request this response indicates that the underlying data has not changed since the previous request, and cached results may be re-used.</td></tr><tr><td><code>400</code></td><td>Bad request</td><td>For example, missing query parameter or malformed syntax.</td></tr><tr><td><code>401</code></td><td>Unauthorized</td><td>The client has not provided authentication or has provided incorrect authentication.</td></tr><tr><td><code>403</code></td><td>Forbidden</td><td>The client has authenticated its access but does not have sufficient rights to complete the request.</td></tr><tr><td><code>404</code></td><td>Not found</td><td>The server has not found anything matching the Request-URI.</td></tr><tr><td><code>405</code></td><td>Method not allowed</td><td>The request used an unsupported HTTP method, for example, <code>DELETE</code> or <code>PUT</code>.</td></tr><tr><td><code>429</code></td><td>Too many requests</td><td>The number of requests per minute (rate-limit) has been exceeded.</td></tr><tr><td><code>500</code></td><td>Internal server error</td><td>A generic internal server error occurred.</td></tr><tr><td><code>503</code></td><td>Service unavailable</td><td>A temporary outage due to overloading or maintenance.</td></tr></tbody></table>


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