An E121 error relates to the indoor unit’s room temperature sensor, the small thermistor that measures the air being drawn back into the unit so it knows how hard to work. When this sensor reports an open circuit or a short circuit instead of a normal reading, the control board can’t accurately judge room temperature and raises the fault.
Because the whole system relies on this sensor to decide when to cycle the compressor and fan, a faulty reading here effectively blinds the unit to the conditions in your room. It’s a common, usually inexpensive fix once identified.
Send us the code and a photo of the display, and a Ford & Doonan technician will get back to you. Please don't reset the unit before you take the photo.
(08) 9331 8800