The board drives the fan and reads its actual speed back through a feedback signal. When the fan does not turn, turns too slowly, or the board gets no feedback, the unit stops and reports a fan error.
What to check, in order
- Power off and turn the fan by hand: it must turn freely, without binding or touching the casing; clean a dusty blower or blade.
- Check the fan motor connector on the board: firmly in, not burnt, not corroded.
- Power on and measure the motor's supply and control signal as the exact model's manual describes (DC and AC motors are measured differently).
- Check the speed feedback: if the fan turns and the board still reports the error, the motor's feedback or the board's input is at fault.
- For the outdoor fan also look for obstructions, wind against the fan, and the fan capacitor on an AC motor.
Safety: switch off both the indoor and the outdoor unit before opening a cover or touching a board; the capacitors on the outdoor board stay charged for a while after power-off - wait for the board's lamps to go out and measure before touching. Refrigerant and high-voltage work belongs to a certified technician.