Cause: Option communications error comes when a communications error occurred between the option card and the inverter. 

er4 fault in fuji FRENIC MEGA drive indicates Option communications error. This means the drive is having trouble communicating with an installed option card or module.

• Check whether the connector on the option card is properly engaged with that of the inverter.
• Check if appropriate noise control measures have been implemented.
• The option card might not be fully seated in its slot on the drive's control board so remove and re-insert again.