How do I clear or fix error 66?

Power down the inverter and remove the cover (only if the unit is field-serviceable per the manual). Inspect the fan for debris and clean.

Felicity inverter error 66 (F66)

Fan 1 Fault

Also displayed as F66 on some models.

Fault — the inverter cuts its output and the red LED stays solid until the fault is cleared.

InverterFault

Fan 1 Fault — Fan 1 is faulty, check whether the fan is normal.

Cause: Internal cooling fan failure detected. Often caused by dust buildup, bearing wear, or debris fouling the fan blades.

Power down the inverter and remove the cover (only if the unit is field-serviceable per the manual).
Inspect the fan for debris and clean.
If the fan does not spin freely or is noisy, replace it with the manufacturer's OEM part.

Source: t-rex-25-50khp3g01-user-guide-en.pdf, p.48

What does error 66 mean on a Felicity inverter?: 66 means: Fan 1 Fault. Internal cooling fan failure detected. Often caused by dust buildup, bearing wear, or debris fouling the fan blades.
Is error 66 a fault or a warning?: 66 is a fault code: the inverter cuts its output and the red LED stays solid until it is cleared.
How do I clear or fix error 66?: Power down the inverter and remove the cover (only if the unit is field-serviceable per the manual). Inspect the fan for debris and clean.

Compiled from official Felicity user manuals. Always consult your device manual and a qualified technician before performing repairs.