It sounds like it does it every time so I would expect either the sensor is bad or something has moved. Hook up a meter to the sensor and wave the beam a few times. If they test good with a meter, you need some way of watching both sensors at the same time. I’m not sure if you can see them with the UIT, I have virtually no experience with that. If not, 2 meters or a couple 24v LEDs should suffice.

If it’s he job is that far away then ya, I’d probably get the part before going out there just in case.