I have seen this situation many times. The photo eye system the issue. Fire a wash output y40 comes on then input x20 should come on. when x20 (photo eye input) goes off, then UC comes on.
Turn on y40 in your diagnostic ,then watch input x20 to see if it is coming on and staying on, or if it is flashing.
The amp is located in the J2 box, you may also want to test the strength of the eyes . turn on y40 again, all 3 lights on the amp should come on (if all 3 do not then you have an eye problem). slowly turn down the potentiometer. the relay light shouldn't go out until around 2 .
If an eye is the issue it is usually the receiver (black wire) but in around 20% of the time it is the transmitter (gray wire).
If the eyes test OK try replacing the amp (very unlikely). Their is also a very slim possibility that the signal wire back to the j0 is bad.
Hope this helps some ,Lag