what does error code 11 mean?

Each brand has it's own error code system. What brand is it.
Chances are it's a speed sensor that is dirty. Remove the motor hood and clean all dust and dirt. Then blow it out.
The speed sensor may be a strobe wheel on the motor or front roller, again it depends on the brand.
http://www.mindbites.com/lesson/377-maintaining-your-home-treadmill-revised

Mark Peterson
May 2008