The internal DSU passes the T4 test, but fails an external loopback test.

Check the four pins on the edge of the internal DSU card to make sure that they are seated in the female socket on the aggregate board. These four pins provide continuity to the RJ48s jack on the back of the mux labeled Internal DSU .

The RJ48s " internal DSU " interface may have been damaged in a power surge or a lightning hit.