This fault indicates that the grid the inverters are measuring is not stable. This can because by several factors. 


  1. Multiple inverters installed in line and the grid voltage wave is non-ideal.
  2. Grid filters need to be set. 
  3. Downstream equipment needs to be changed or repaired.
  4. Power board is faulty.


This fault is difficult to troubleshoot, in most cases the issue is a faulty power board and will require an RMA. Please check that the inverters are installed properly according to the installation manual, and verify that no other non UL loads or equipment are attached to the home or facilities AC distribution panel.