https://devzone.nordicsemi.com/f/nordic-q-a/66372/nrf9160-mqtt-connection-failureHi, Our application uses MQTT over TLS to talk to a MQTT broker. The MQTT APIs used are like in the &#39;mqtt_simple&#39; sample code. When mqtt_connect is called with invalid client ID, we see that connect call is successful and we get MQTT event - MQTT_EVT_CONNACKen-USTelligent Community 13Tue, 29 Sep 2020 16:11:15 GMThttps://devzone.nordicsemi.com/thread/272126?ContentTypeID=1Tue, 29 Sep 2020 16:11:15 GMT137ad170-7792-4731-bb38-c0d22fbe4515:43345927-5da7-4d06-bf48-3e1d78ed1e30Heidi<p>Hi!</p> <p></p> <p>No, closing the socket directly should not be necessary.</p> <p></p> <p>MQTT event MQTT_EVT_CONNACK with result code 4 means the MQTT broker rejected the client. So after the event is handled (the event handler return) the socket will be closed automatically by the library.</p> <p></p> <p>However, if you are trying to re-connect in the event handler, right after receiving MQTT_EVT_CONNACK, this will not work, as the socket will not be closed by the library.&nbsp;</p> <p></p> <p>Try letting the event handler return before attempting to call mqtt_connect again.&nbsp;For instance, set a flag in the event handler and then verify it in a loop from the main thread. One of the samples in Zephyr does something like this, see <a href="https://github.com/zephyrproject-rtos/zephyr/blob/master/samples/net/mqtt_publisher/src/main.c#L364">here</a>.</p> <div></div> <p>If you are still having trouble, it would be nice if you could provide&nbsp;some sample code from your application. It is much easier to find potential mistakes this way. I can also make the ticket private, at your request, if you would prefer to keep your code private.</p> <p></p> <p></p> <p>Best regards,</p> <p>Heidi</p><div style="clear:both;"></div>