Where can I get a manual for Litter Robot iii /

sensor is not working and I dont know where it is

Hello @vinifera and welcome to the forum!

Here you can find manual for Litter Robot III: https://www.litter-robot.com/manual_pdf/Litter-Robot_III-Manual-Revision_C.pdf

But please share more info about your problem with us. Which sensor you think is not working? Cat sensor maybe?