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Saturday, September 17, 2011

When try to connect to Internet USB Modem (Huawei/ZTE/LG/Nokia), it gives error: “Unable to open serial port” or “Unable to open COM port for USB Modem”


1.  After installation USB Modem (Huawei/ZTE/LG/Nokia) works good, but when it is run after a restart of Windows XP/Vista/Windows 7, it gives error “Unable to open serial port” or “Unable to open COM port for USB Modem”. When USB Modem is plugged-out and plugged-in again, the problem gets resolved, and starts functioning again.
2.      The USB modem is detected without any problem.
3.      Query Modem” feature from Device Manager > Modems > XXXX USB Modem > Diagnostics > Query Modem is working successfully.

The Problem

Possible reason of the problems is that there is conflict of Serial Ports (COM__) between the USB Modem and applications already installed on the Windows system.


One of the solutions for this problem is to manually assign the free and higher order Serial Ports (COM__) to the USB Modem:
1.      Select My Computer, right-click and click to Properties:

2.      Go to Hardware tab and click Device Manager button:

3.      In Device Manager, select the Ports, and find your USB Modem, right-click it and select
Properties, for example, for ZTE USB Modem:

4.      In USB Modem Properties, got to the tab “Port Settings” and click “Advanced” button:

5.      Now, from the drop down list “COM Port Number”, select a higher order and not in use serial port, for example, in the given picture below, COM71onwards can be selected.

6.      Do the same procedure for all the COM ports used by the USB Modem. For example: for ZTE Diagnostics Port, ZTE GPS Port, ZTE Service Port, and ZTE Voice Port set COM71, COM 72, COM73, and COM74.


  1. Thanks mate for sharing this great information. I'm having the same problem and after following these steps, I'm getting the same error. What should i do? Why do we've to choose this COM port71? Are these specifically for different type of USB modems? Please look forward to solve my problem

  2. In my case, the Serial Ports (COM__) which the USB Modem was trying to use were already occupied by applications installed on Windows. so, I tried to assign higher order (vacant)serial ports to Modem. COM port71 is not mandatory. You can try any other higher order ports such as COM80,....