In our testing there have been occurrences where the jaw was locked closed. In order to open the clamp in order to put a tool in, the air pressure needs to be relieved. 

The symptom of this was the inability to utilize the MTC button to open it therefore the ATC wouldn't force it open either, and the reading of ~> .6ma according to the gauge. 

It has been duplicated at a point of failing to grab a tool. To rectify the error, release the air line on the tool port either at the spindle or at the pneumatics enclosure. This will release the pressure on the line at would be noted at by watching the gauge on the PE. 

To further limit this check your clearances with the ATC and ensure that it is programmed to be low enough to reach the tool.