F4
Fraser-Johnston
Outdoor Coil Temperature Sensor Fault / Defrost Sensor Error
F4 error, outdoor unit shuts down in heating mode
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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
Failed outdoor coil thermistor, Sensor not properly attached to coil, Damaged sensor harness, Faulty defrost control on outdoor PCB
How to Fix / Troubleshooting
Safety: Disconnect power at the outdoor disconnect.
- Locate coil sensor: On Fraser‑Johnston heat pump condensers, the defrost/coil sensor is clipped to the outdoor coil tubing.
- Check attachment: Ensure the sensor is firmly clipped and making good contact with the coil. Re-secure if loose.
- Inspect wiring: Follow the sensor wires back to the control board. Repair any damaged insulation or broken conductors.
- Test sensor: Measure resistance and compare to the temperature chart. Replace if out of spec.
- Defrost board: If sensor is good but F4 persists, the defrost control function on the outdoor PCB may be faulty and require board replacement.
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Repair Difficulty
Medium
3/5
Required Part
Outdoor Coil Thermistor (Defrost Sensor)
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