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Upload error · port

Fix “avrdude: ser_open(): can't open device \\.\COM3: The system cannot find the file specified”

The COM port selected in the IDE doesn't exist any more. Replug the board, re-select the port, and check the driver and cable.

Arduino UnoArduino NanoArduino Mega 2560Arduino Pro MiniUpdated 3 Nov 2026

Community draft — report a mistake

What you see

OutputOutput
Sketch uses 924 bytes (2%) of program storage space. Maximum is 32256 bytes.
Global variables use 9 bytes (0%) of dynamic memory, leaving 2039 bytes for local variables. Maximum is 2048 bytes.
avrdude: ser_open(): can't open device "\\.\COM3": The system cannot find the file specified.

Failed uploading: uploading error: exit status 1

avrdude was told to open COM3, and Windows says no such port exists. The board is unplugged, on a different port number, or not recognised as a serial device at all.

Re-select the port after every replug. If no port appears, it’s the cable or driver, not avrdude.

Quick fix — try in order

  1. 1

    Re-select the port after plugging in

    Tools → Port. The number may have changed (COM3 → COM5) after a replug, a different USB socket or a driver update.

  2. 2

    Watch which port appears

    Unplug the board, open Tools → Port, plug it back in, and see which entry is new. That is your board.

  3. 3

    No new port at all? Fix cable or driver

    A charge-only cable or a missing CH340 driver means Windows never creates a port. See COM port not showing and CH340 driver not installed.

  4. 4

    Port disappears during upload

    If it was there a moment ago, the board reset and dropped off USB — a short circuit on 5 V, a motor or relay on the board's supply, or a worn USB socket.

Why it happens

“Access is denied” instead

Then the port exists but another program has it open. Close Serial Monitor and other serial apps — see port busy / access denied.

Leonardo, Pro Micro

These boards change COM number between sketch and bootloader. The IDE handles it, but if your sketch crashed, double-tap RESET and upload quickly — see butterfly_recv.

Board-specific notes

Arduino Uno R3 (genuine): Shows as “Arduino Uno (COMx)” with no extra driver on Windows 10/11. Clones show as “USB-SERIAL CH340 (COMx)”.

Drivers — official sources only

CP210x (Silicon Labs)silabs.comCH340 / CH341 (WCH)wch-ic.com

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