F4
Fujitsu
Outdoor Ambient Temperature Sensor Fault
Indoor unit displays F4, unit may not heat properly
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Open or shorted outdoor ambient thermistor, Sensor dislodged from mounting point, Damaged sensor wiring, Faulty outdoor PCB sensor input
How to Fix / Troubleshooting
Safety first: Isolate power at the outdoor disconnect before accessing sensors.
- Locate ambient sensor: The outdoor ambient thermistor is usually mounted near the air intake grille, away from direct sunlight and compressor heat.
- Check mounting: Ensure the sensor is properly clipped or screwed in place and not dangling inside the unit.
- Inspect wiring: Look for damage from UV, animals, or vibration. Repair or replace the harness if needed.
- Resistance test: With the sensor unplugged, measure resistance at outdoor ambient temperature and compare to Fujitsu's chart. Replace if out of range.
- PCB check: If a new sensor does not clear F4, the outdoor PCB may have a defective sensor input and should be replaced by a technician.
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Repair Difficulty
Medium
3/5
Required Part
Outdoor Ambient Temperature Thermistor
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