# Failed Serial Transmission Arduino-Node RED: COM4 Access Denied

**URL:** <https://discourse.nodered.org/t/failed-serial-transmission-arduino-node-red-com4-access-denied/30481>\
**Category:** General\
**Created:** [21 July 2020 13:46 UTC](https://discourse.nodered.org/t/failed-serial-transmission-arduino-node-red-com4-access-denied/30481 "2020-07-21T13:46:21Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![MarkParents](https://avatars.discourse-cdn.com/v4/letter/m/dbc845/32.png) [@MarkParents](https://discourse.nodered.org/u/MarkParents)\
**Post date:** [21 July 2020 13:46 UTC](https://discourse.nodered.org/t/failed-serial-transmission-arduino-node-red-com4-access-denied/30481/1 "2020-07-21T13:46:21Z")

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Hi everyone. I'm struggling with this problem: I have programmed my Arduino Uno for trasmitting via USB Serial communication the temperature measured by a temperature sensor connected to one of the Analog inputs of the Arduino board. The port used for serial transmission is COM4. Opening the Serial Monitor of Arduino, I have verified the Arduino-side serial transmission occurs correctly.  
I'd like to display the transmitted info into the debug of Node Red: I've used and connected "Serial In" and "debug" nodes between them. The problem is that when I deploy everything, "serial port COM4 error: Error: Opening COM4: Access denied" error displays. I can't understand what the problem is and how to solve it.  
Thanks to who will try to help me!

 ![image](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/c/9/c92ebaeba01c0de96966d566801d897b15be42bc.png)

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**Author:** ![edje11](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/edje11/32/572_2.png) [@edje11](https://discourse.nodered.org/u/edje11)\
**Post date:** [21 July 2020 14:17 UTC](https://discourse.nodered.org/t/failed-serial-transmission-arduino-node-red-com4-access-denied/30481/2 "2020-07-21T14:17:51Z")

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Did you close the Arduino ide?  
Verify if this is the correct comport in Windows device manager.

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**Author:** ![MarkParents](https://avatars.discourse-cdn.com/v4/letter/m/dbc845/32.png) [@MarkParents](https://discourse.nodered.org/u/MarkParents)\
**Post date:** [21 July 2020 14:25 UTC](https://discourse.nodered.org/t/failed-serial-transmission-arduino-node-red-com4-access-denied/30481/3 "2020-07-21T14:25:38Z")

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I forgot to write it. Anyway yes, I closed the Arduino Ide.  
After programming Arduino, I first closed the Arduino IDE and then I launched Node RED.

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**Author:** ![system](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/1X/d073cd938eafa2e558d7c2cd59003b3ef4963033.png) [@system](https://discourse.nodered.org/u/system)\
**Post date:** [19 September 2020 14:25 UTC](https://discourse.nodered.org/t/failed-serial-transmission-arduino-node-red-com4-access-denied/30481/4 "2020-09-19T14:25:45Z")

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