More information at the above url and copy&pasted below, but it appears that the 103-1003 error code is due to the Wii U is unable to find the wireless router or access point during the test connection. and/or if you have entered an incorrect WEP key into the Wii U connection settings.
But if the troubleshooting tips do not help, probably it would be best to give a call to Nintendo's 1-800-255-3700 (6 am ~ 7 pm Pacific, everyday) Customer Service number,and one of the service representatives will be able to some further over the phone troubleshooting and talk you through any needed steps to correct this.
What to do:
Changing your router's wireless channel to 1 or 11 may also help increase signal strength. [How to]
Still Not Resolved:
If the above steps did not resolve the issue, and you continue to experience problems, please click the button below to see which options are available to contact us for further assistance.