Means that it is broken usually, but it is controlled by the TTV. Check my FAQ post all will be revealed
http://www.golfgtiforum.co.uk/index.php?topic=18871.0Edit: Like you say you need to take the carb off to change it:
Remove the 3 big bolts on the top of the carb
Unplug all electrical connections, 2 plugs on the left (ttv and idle overrun), an earth lead on top and another plug on the right (autochoke).
Unplug the vacuum connection from the inlet manifold to the pull down unit
Remove the fuel line at the front
Remove the throttle cable
It should then come off
