NEC NC1600C Service Manual page 120

Interlock Fail
Is there no problem in external control?
(There is occasionally interference with an external
interlock.)
Yes
Look for any damage or the drawing out of the
connector and follow the steps of replacement
work in the order specified below. Replace them,
as required.
PEDE-A PWB
CPU PWB
"Confidential, Do Not Duplicate without written authorization from NEC."
TROUBLESHOOTING
No
4-25
Normal operation