If a M51 or M52 is issued, does either of the relays (outputs Y48.Y48.5) operate?
In some cases an issue of an M code requires a confirmation by way of a FIN(ish) signal back to the PLC input.
If the issue of an Mcode freezes, either the M code is not written or the FIN signal never comes back.
The ladder rungs should show if the outputs are activated.
Al.