The switch contacts burn out or get pitted and won't make good contact, you can open the switch and clean the contacts. This is easier said than done as you will need to open the handel part of the unit. 1 where mains cable enters pull out the black bottom piece which is gel in by 3 clips. 2 open the 2 small screws 3 you will need to pull out the cold button, it has a read copper metal on it which is soft and easily damaged. Best I find is to use a very small screw driver tip ( about 1mm) and push it down the side of cold button and try to leaver it off , it is held in by two plastic clips at the top and bottom. 4 the back of the handle lift off , there are 6 plastic blades that hold in place so lift it up pass the on and off switch. Jam something in it to keep it up as far up as possible the lift off the copper on and off switch slider up and out. 5 now the whole havdel would slide down over the power cabel. 6 the clearnplastic parts are held together with 2 philps screws, open and remove the bottom part....