This is a pretty difficult model to work on because the bonnet nut is so slim. The way I do it is to dismantle the faucet from the sink. Remove the cap and handle (2mm set screw) and lay the faucet on it's side. Protect the finish by putting it on a towel and lay it on a firm surface such as a table or work bench. Leaving the body and spout on the table, hang the top of the faucet over the edge of the work surface. Carefully lock vice grips onto the nut. Be careful to use just enough tension to secure it to the nut. Dont go too hard, or you will just chew up the nut. Then lightly tap the channel locks with a small hammer or mallet to turn the nut counter clockwise. Do this while pressing down on the faucet body to keep it from jumping. Lots of little taps will loosen the nut which is locked in by mineral deposits from leaking. Once the nut is moving you no longer need to tap it, as it can then be turned by using your hand to turn the vise grip pliars.
Look for an old o-ring in the nut where it meets the top of the cartridge and replace it with the supplied new one. Also replace the two seat washers that sit in the faucet body at the base of the cartridge with the supplied new ones. The newer cartridge will be smaller diameter, but you will see that the height is the same and it works perfectly.
Lastly, if this is all too difficult, Hansgrohe will allow you to send your faucet in under the warranty. We will repair, test, and send it back to you. You can call us at 800-334-0455 for service in USA, or contact us through the web site www.hansgrohe-usa.com