How to fix Arduino Uno COM3 error?

 On Windows, a “COM3 error” with Arduino Uno usually means the PC can’t talk to the board on that serial port. Let’s go step by step from easiest to deeper fixes.

I’ll assume you’re on Windows and using the Arduino IDE; if not, tell me your OS and I’ll adapt.



1. Basic checks (do these first)

  1. Unplug and replug the USB cable

    • Use a different USB port on the PC.

    • If possible, try a different USB cable (data cable, not a charge-only phone cable).

  2. Close anything else using COM ports

    • Serial Monitor from another IDE

    • Terminals like PuTTY, Tera Term, CoolTerm

    • Manufacturer tools (ST-Link Utility, other dev tools)
      If another program has COM3 open, Arduino can’t use it.

  3. Restart the Arduino IDE

    • Close IDE → reconnect Uno → then reopen IDE.

2. Make sure the right port is selected

In Arduino IDE:

  1. Go to Tools → Port

  2. Look for something like:

    • COM3 (Arduino Uno) or

    • COMx (Arduino Uno) if it changed.

  3. Select the one that says Arduino Uno.

If Tools → Port is greyed out, Windows doesn’t see the board at all → jump to section 4 (driver & Device Manager).

3. Make sure the correct board is selected

In Tools → Board choose:

  • “Arduino Uno” (or “Arduino Uno (Old Bootloader)” only if it’s a very old clone and uploads fail with normal Uno).

Wrong board+port combination can give COM/timeout-style errors.

4. Check Windows Device Manager

  1. Right-click Start → Device Manager.

  2. Expand Ports (COM & LPT).

    • If you see “Arduino Uno (COM3)” or “USB-SERIAL CH340 (COM3)”, note that COM number.

    • If COM3 doesn’t show up when you plug the board:

      • Check under Other devices or Universal Serial Bus controllers for anything with a yellow exclamation mark (unknown device).

Cases:

✅ Case A: It appears as something like “Arduino Uno (COM3)”

  • Good: the driver is okay.

  • If the IDE still shows COM error:

    • Try another port number:

      • Right-click the device → Properties → Port Settings → Advanced → change COM number to e.g. COM4 or COM5 → OK.

    • Reopen Arduino IDE and select the new COM in Tools → Port.

⚠ Case B: It appears as “Unknown device” or something with yellow “!” mark

This means driver problem.

  • Right-click → Update driver

    • “Search automatically for drivers” often works if you have internet.

  • Or install drivers from the Arduino IDE folder:

    • For “official” Uno:

      • Inside Arduino install directory: drivers → run dpinst-amd64.exe (64-bit) or dpinst-x86.exe (32-bit).

    • For CH340 clone boards:

      • You need the CH340 driver (search “CH340 USB driver Windows”). After installing, replug the board and you should see USB-SERIAL CH340 (COMx).

5. Common specific error messages & fixes

a) “avrdude: ser_open(): can’t open device \.\COM3: Access is denied.”

Often means another program is using COM3.

  • Close:

    • Arduino Serial Monitor

    • Any terminal programs

    • Any other MCU tools

  • Unplug & replug the Uno

  • In Device Manager, you can also:

    • Right-click COM3 → Disable then Enable (forces a reset).

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

This usually means the port changed or disappeared.

  • Open Device Manager.

  • See which COM port the Uno is on now (maybe COM4 instead of COM3).

  • In Arduino IDE: Tools → Port → select that COM instead of COM3.

c) It used to work, now nothing shows up

Possible causes:

  • USB cable damaged → try another.

  • USB port damaged → try different port or another PC.

  • Board’s USB interface damaged (fallen off desk, ESD, etc.).

    • If the power LED doesn’t turn on at all → board may be dead.

    • If power LED is on but PC never detects it (no sound, no COM device) → USB-to-serial chip might be damaged.

6. Try a different PC if possible

If another computer instantly recognizes the Uno:

  • The board and cable are OK.

  • Then the problem is Windows drivers/ports on your first PC → focus on uninstalling/reinstalling drivers there.

On the problematic PC you can try:

  1. Device Manager → right-click the Arduino COM device or unknown device → Uninstall device (check “delete driver software” if present).

  2. Unplug Arduino.

  3. Reboot Windows.

  4. Plug Arduino back in and let Windows reinstall drivers (or reinstall Arduino IDE which includes drivers).

7. Quick checklist you can follow

  1. Change USB cable & port.

  2. Check Tools → Board = Arduino Uno.

  3. Check Tools → Port = the COMx that shows up/disappears when you plug/unplug the Uno.

  4. Look in Device Manager:

    • If COM3 looks okay → maybe change it to another COM number.

    • If “Unknown device” → reinstall driver (Arduino or CH340).

  5. Close all other serial programs.

  6. If still broken, test on a second PC to see if the board itself is okay.

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