The FINDA sensor, sometimes referred to as F.I.N.D.A. is essentially the MK2/S PINDA sensor (without thermistor), upcycled to be used as a filament sensor. It is triggered by the proximity of the stainless steel ball. When the filament is pushed through the selector, it raises the ball towards the FINDA. There is a small red LED light on the top of the sensor that turns OFF when the filament is detected. When the filament is removed, the stainless steel ball falls back down and the red light turns ON.
The FINDA sensor must be distanced correctly from the steel ball (blue ruler - left picture) so it is triggered only when filament is present (green arrow - right picture), pushing the steel ball higher.
The FINDA sensor has to be properly inserted into its socket: not too low, not too high. Check out our assembly manual, specifically this step, to see how the FINDA fits in.
Finally, tighten the M3x10 screw to prevent the FINDA from moving. The steel ball, however, has to move freely!
As explained in the first paragraph, the steel ball has to move freely. Sometimes, however, filament debris, dust, or strings may amass in the hole or even in the FINDA/ball socket. Use compressed air to clean it.
You can always check the proper functioning of the sensor by inserting a piece of filament and observing the red light on top of the sensor. You can also go to the LCD Menu -> Support -> Sensor info to see the FINDA status. It should be 0 when the filament is out (red LED on) and 1 when the filament is inserted into the selector (red LED off).
In some situations, the FINDA itself may be working but its cable might be damaged and fail to transfer the information to the logic board. In most cases, it can be damaged during the assembly while doing cable management. It can be also damaged by opening the Idler body all the way and having it hang on the cables.
The best way to check the cable is to insert a piece of filament into the selector and go to the Sensor info menu. The FINDA value should be 1. Then wiggle with the cable and see if the value flickers and changes to 0 at any point.
In case the FINDA value in the Sensor info menu is N/A during manual testing, check the cable connection to the MMU logic board and the MMU cable connection into the RAMBo board (MK3S / MK2.5S). Pay attention to the order of the colors of the individual wires. However, during load and unload, the FINDA value in the Sensor info menu will be N/A.