> For the complete documentation index, see [llms.txt](https://guides.clickatell.com/flow/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://guides.clickatell.com/flow/system-error-codes/system-error-codes.md).

# System Error Codes

This section provides insight into Chat Flow's system error codes. Each of these error codes are associated with a cause, providing flow-builders the ability to debug flows.

## System Terminology

| Item             | Description                                                                                                                                                |
| ---------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------- |
| Engine           | Chat Flow back-end; controls transportation of inbound & outbound content                                                                                  |
| Gateway          |                                                                                                                                                            |
| Flow Config      | This represents issues relating to flows being incomplete or incorrectly built                                                                             |
| WhatsApp Adapter | Channel adapter for the WhatsApp channel; controls inbound & outbound transportation of messages between the user and the workflows developed on Chat Flow |

## Error Codes

| Code | Description             | Cause                                                            | Location/Service        |
| ---- | ----------------------- | ---------------------------------------------------------------- | ----------------------- |
| CT01 | Non Recoverable Error   | Error constructing response in Engine, or next step is not found | Engine                  |
| CT02 | Non Recoverable Error   | Session is missing when preparing response in Flow Engine        | Engine                  |
| CT03 | Generic Gateway Error   | Unhandled error on incoming request to Gateway                   | Gateway                 |
| CT04 | Missing Next Step       | Next step missing in flow                                        | Flow Config             |
| CT11 | Read Timeout            | Timeout between our services                                     | Gateway                 |
| CT12 | Read Timeout            | Timeout between our services                                     | Engine                  |
| CT21 | Unhandled Error         | Unhandled Error Occurred                                         | Engine                  |
| CT22 | Unhandled Error         | Unhandled Error Occurred                                         | WhatsApp Adapter        |
| CT40 | Session Closed          | Session being referenced has already been terminated             | Gateway                 |
| CT51 | Unhandled Http Response | Unexpected/Unhandled response. i.e. missing "other" case in flow | Flow Engine/Flow Config |