text: <s>[INST] Powerflex 700 Fault code 121 F121 I/O Comm Loss- Manual points to possible induced noise on I/O wiring, but O-scope did not detect any issues. I/O card has been replaced and still getting this fault. We then tried isolation relays on the input wiring and still getting this fault. Does anyone have any ideas????? [/INST] Aid 21123 states this fault is caused by the I/O board (or its processor) not communicating to the main control board (or its processor).It also suggest to replace the I/O board, as you've already done without success.... :smileysad: The PF700 User Manual also reports there could be the need to replace the Main Control Board.... I/O Comm Loss F121 - I/O Board lost communications with the Main Control Board. Check connector. Check for induced noise. Replace I/O board or Main Control Board. - fuzzy logic \\n </s>