I had a procedure that worked successfully (2005 MCS) the last time this happened, and I cannot find it on the forum or on the web. The light comes on after an extended period of cold weather; something to do with the temperature of the air entering the turbo. Anyway, all is fine, and I'd like to reset the light.

 

Depending on what kind of fault it is you can sometimes reset it by disconnecting the battery, but if that doesn't work you will have to have a tool.