I am having the same alarm on our VQC. I HAVEN'T FIXED IT YET, but I have an idea on where to go.
I presume that you are getting this error the same time I do - when you want to change a tool. In my case the tool drum door comes open, the spindle moves close (its home position in Y?), and then generates the error.
From the description in the book, it looks like the control is looking for a tool to be in the spindle, but doesn't see one, even if there is actually one there.
The optical sensor to detect this is PHS-182. You really have to look to find it. Open the magazine cover door, and look (with E-stop engaged) into the magazine area, with your head right where the spindle tool would be. Look above the rotating drum that holds the tools. You will see that a radial sheet metal shield almost touches the upper flat sheet metal cover, except for an opening, about 6" wide. Look through that opening, and you should be able to see the sensor. It looks rectangular, long axis vertical, with what looks like two plastic lenses (about the size of a pencil eraser) one above the other. I am presuming that this sensor sends out a beam which is aimed at where a tool would be, and is reflected back into the other sensor. Thats my theory, at any rate. How to get access to the sensor is something I have not yet figured out.
This sensor ends up driving relay R17 (SPDT) on the relay card. This should give you enough info to at least start chasing down the problem