The ESP32-OTA-Pull library defines the following error codes in its ErrorCode enum (returned by CheckForOTAUpdate()):

UPDATE_AVAILABLE (-3)
An update matching your board/device/config was found in the JSON, but you passed DONT_DO_UPDATE so no firmware download was performed.

NO_UPDATE_PROFILE_FOUND (-2)
No entry in the JSON’s Configurations array matched your board, device, or config strings.

NO_UPDATE_AVAILABLE (-1)
A matching profile was found, but its "Version" wasn’t newer than your current version (and downgrades weren’t allowed).

UPDATE_OK (0)
The update was downloaded, written to flash successfully, and (if not suppressed) the ESP32 was restarted.

HTTP_FAILED (1)
The HTTP GET for either the JSON descriptor or the binary returned a non-positive code (e.g. couldn’t connect).

WRITE_ERROR (2)
A mismatch occurred between bytes read from the server and bytes written to flash during the OTA process.

JSON_PROBLEM (3)
The library failed to parse the JSON descriptor you provided (malformed or too large).

OTA_UPDATE_FAIL (4)
The actual OTA sequence (calling Update.begin() or Update.write()) failed before completion.

Additionally, if the JSON download returns a positive HTTP status other than 200 (for example 404 or 500), CheckForOTAUpdate() will return that raw HTTP code instead of one of the above