Serial Port 'Com4' already in use

When I try to upload Blink to my Duemilanova, I get the Serial Port 'Com4' error. It suggests that I quit any programs using 'com4'.

I am sure that I don't have another program using 'com4'.

This is version 18 of the Arduino IDE. I am running Win2K with Service Pack 4. I have tried reloading the USB drivers without any change. I have also tried the reset button tricks that have been suggested, but that has not worked either.

The L led blinks with an uneven pattern, which I think is the test code that was left on the Duemilanova.

Thanks for your assistance. JHoward

Does the computer have a mobile phone application installed?

You possibly have some other communication app open. Phone stuff etc can mess this up.

Try going into Ctrl-Alt-Del, right clicking on explorer.exe and ending program tree. Then start new task explorer.exe and see if it works then. Just as a test.


When I use WinXP, if I have the on board serial ports enabled in the bios I get this error between the onboard com1 and the usb com3. I had to disable the onboard serial to cure it.

Maybe it would also work if you changed the USB virtual COM port's COM port number (can be done in device manager).


Here is some additional information. The IDE tries this 3 times before giving up.
There are two almost identical messages just before the one complaining about Com 4 being used:
as ( 144)
as (

Can anybody give an idea as to why there are two of these?

I do not have any mobile phone apps on this computer. I do have a SanDisk Sensa, but it is not connected.

Does the IDE use a web browser, and in particular, Microsoft Explorer? I generally used FireFox for the web, and the version of MS Explorer is version 6. If it is used, maybe it is too old.

Now I have to try a couple of your suggestions.

Does the IDE use a web browser


I do have a SanDisk Sensa, but it is not connected.

That would not cause an issue.

Can anybody give an idea as to why there are two of these?

Nope 'fraid not.


The FTDI driver allowed for COM4-COM7. When I selected a different com port, the system said that they were already in use, except for COM4.

Then I looked at the FTDI driver that was being used, and it was dated 2001, clearly an old driver. I updated the driver with the one supplied with the Arduino IDE, and now everything works.

Thanks to all of you who made suggestions for things that I should try. JHoward