Tray moving down instead of up on ”Home”

Hi,
I am still in preparation mode. I haven’t printed anything yet.
Calibrating the tray, choosing ”home” from the panel. Instead of going up into a top position, the tray goes down approx 1 cm. This happens each time.
Any idea why the tray is going the wrong way?

Check the endstop for the Z axis (back of the machine.
It is either dirty or defective