Rockwell Automation 1772-LV Mini-PLC - 2/15 Programmable Controller (Series B) Programming and Operations - Manual (Page 169)

Troubleshooting Aids
Chapter 13
13Ć5
If an illegal opcode should occur you can compare the rung containing the
illegal opcode with the equivalent rung in a hard copy printout of the program.
you must either replace the error with its correct instruction, replace the
instruction, or remove it. The ERR message due to an illegal opcode cannot be
removed directly. Instead, remove and replace the entire rung. You should
identify and correct the cause of the problem in addition to correcting the ERR
message.