I'm posting some tips from eureka:
 
If the wrong MCU PWB was installed in the machine, fault code 124-315 will be declared.
If the Serial Number Synchronization Procedure (GP 4) is run two times, the machine will be locked up in a 124-314 condition and the only way to recover without replacing the machine is to call Field Engineering to open a log indicating which machine it is (DC240/242/250/252/260), then send the original (or correct - new) MCU PWB, MCU NVM PWB (PL 11.1, items 12 & 14) and Control Housing Assembly (PL 11.5, item 1)  to Field Engineering.
 
Also send a copy of the NVM SETTING VALUE LIST that is in tray 1.
 
The Program Team will synchronize the serial number and speed then return the PWB's.
 
Field Engineering note: After receiving the PWB's, contact Rich Johnson to synchronize serial numbers and machine speed.

and second

Customer had replaced fuser unit,after switching power back on machine displayed 124-315 code IOT speed mismatch fault, also displaying secondary code of 10-324.

Cause: Suspect NVM corruption

RAP not clear (only one serial number mismatch).
GP4 would initially clear 124-315 code but it would always re-appear.
Replace MCU pwb and MCU NVM pwb reload software and restore NVM manually from factory sheets.
This fault also caused other problems with consumables 91-925,93-926,93-940 codes.
Suspect damage or corruption to crum chips. Replaced relevant consumables as spare crum chips were not readily available.

and third

124-315 following by a 61-319

Cause: NVM corruption

after 124-315 the RAP tells you to do a GP4 , I did it but get a new fault 61-319.
 
Make in order DC351 SAVE MACHINE SETTING , RELOAD THE SOFTWARE , ,MAKE A DC 301 TO INITIALIZE THE IOT ONLY ,MAKE A DC351 TO RESTORE THE MACHINE SETTING


Everything points to NVM coruption so try with reseating the NVM pwb. And then maybe NVM init.
Hope these helps.

I hope these helps.