Explanation of Issue
If the mains in voltage reads normal to the CEM3, there may be two possible causes (in order of possibility):
Component failure
As is the nature of power components, components can fail in time. Power surges, spikes, brownouts, and anything else that affects mains power can affect the input side of components on the CEM3. There are several components per phase which can cause an overvoltage error to be thrown when they fail, none of which are repairable in the field
Lost Calibration
Each phase calibration is stored in DSP memory. It is unknown what could cause calibration memory to be lost, but it does rarely occur. Calibration cannot be set in the field