Encoder sensor is at the very back of the carriage. The ribbon cables for the heads make it difficult to see and get to.
To get at mine I turn the printers main power switch off and unplug the ether cable.
Then I walk away to let power "drain" from the electronics.
If you are extremely careful and patient you don't need to remove the ribbon cables. Just undo the big clip at the top and gently move just enough to get your screwdriver past.
One thing I will recommend is a very lightly magnetized screwdriver. This will keep you from dropping the screw into the guts of the machine.
I also recommend one of these for reinstalling the screw.
http://www.matcotools.com/catalog/product/SS145/UNIVERSAL-SCREW-STARTER/