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  1. Folks, we have a set of Mitsubishi Q12PRHCPU (redundant) running effortless since many years. CPU-A as standby and CPU-B as Control. Recently we noticed BATT low alarm on CPU-B. So we arranged for a spare battery Q6BAT and started the procedure as per manual(attached procedure and flow chart). We did memory copy from Control to standby (as prescribed precaution) and did the Reset of Standby PLC CPU-A. Memory copy from B to A completed successfully 100%. But the BACKUP LED did not turn Green from Red & Error LED kept flickering on standby CPU-A. CPU-B remain in Control but backup LED turned RED. From system monitoring we found that there is a Error 6000 (FILE DIFF.) observed in One of the .QPG (program 00-MDBUS.QPG). We tried to reset it but were not successful. We are surprised that why does the program differs when it was copied from CPU-B to CPU-A using the memory copy. Still we did the VERIFY and found no difference between the programs in CPU-A & CPU-B.  Can anyone explain the cause of this issue and the solution. Please note that the CPU-B is still in control and plant is running. Backup LED on both CPU are red which means (redundancy loss) and error LED on standby CPU-A is flickering RED. There is no error in the CPU-A (Control) -we checked this using the GX Developer. Also attached here are the image of the error and error help. Thanks in advance. Battery Replacement procedure.pdf Battery Replacement procedure-Flow Chart.pdf Error.pdf MELSEC QCPU ERROR CODE 6000 FILE DIFF.pdf