How can I fix my Dyson DC 04 Absolute +?

If you look at the switch button from above, there is a flat on the side nearest the main body. You will see a cut out roughly 2mm wide, in the main body not the switch button. You need to push something thin down the opening between the body and the button to disengage the barb and allow the button to be lifted out. The switch will be visible and it's just a case or removing the screw.
Should be staight forward after that.

Note. If you break the barb off the button while trying to prise it out, it's not the end of the world. It will sit in there OK and still work. I know this because I found out the hard way!

paul
June 2007