AC works some of the time then air handler fan stops working. To protect compressor I then turn off AC. I try turning it on again every few hours until it works.
About 16-24 hours later turning AC on again may cause fan to restart and AC works fine again for 1/2 day to 8 days.
Sometimes the fan tries to restart but cant get going. Speeds up then turns off, repeats this cycle 4 or 5 times, then stops.
Problem is severe now (Arizona summer). Problem started at tail end of winter season when heat pump was in heating mode.
Thermostat was changed but made no difference.
When not working manually switching on fan at thermostat causes relay to click on air handler interface board, but fan stays off.
24v ac measured between R & C on control board. 0.36v between R & G when switch in fan on mode. Both seem correct.
Air handler control board has an LED that flashes correctly to indicate board is working.
Disconnecting thermistor wires causes control board to correctly flash at a different rate.
Measured resistance of thermistor, seems to be correct for the ambient temperature.
Interface board also has a LED to flash the CFM rate. When AC not working LED can be seen flickering in dim light.
Suspected interface board so disconnected then reconnected cables to check for bad connections. It may have helped a little for a while, but maybe wishful thinking.
Fan runs freely, no unusual noises, no jerkiness when fan switch on attempted.
Called out AC repair technician but unit decided to work when he came!, and repeated on/off attempts did not reproduce the problem. So no fault detection possible. Technician suspects interface board or bad motor.
Compressor fan capacitor replaced last year to fix compressor fan problem. Compressor works fine now.
So to avoid another wasted service charge does anyone have any further ideas? Many thanks in advance for your help.