It's located on the right side (the one with the flaps). Steps:
1. Remove right covering. Just one screw at the back.
2. Remove big Fan with three screws and a plug.
3. Now You will see the housing of the sensor wheel. Unplug the the cables in the middle (red, white, 2 x purple and black). Plugs are not in the picture. Then carefully pull them out of the hooks. Now You can open the five clips and remove the lid. It's the whole semi-transparent thing.
4. Now You can see the sensor wheel. It's the plastic disc with the text „REV 2.0“ etc. on it. If You look very closely, You will see, that the thick gray line on the edge consists of tiny lines. If there is oil on them, the optical sensor (black box on 7:30 position) can not differentiate them any more. It's easy to clean them on the outside. I used Isopropyl Alcohol.
Attention: The x-shaped heating item in the middle (where the cables go to) has turned and You will have to remeber that when reassembling.
5. To clean the wheel on the inner side You need to remove the optical sensor by removing the metal clip.
6. When reassemble the lid it's difficult to get the cables in the middle in the right order as well as the heatig unit which has turned. Please refer to picture 3.
