F5
Ideal
Return Thermistor Fault
Heater locks out with F5, often after long operation or high demand.
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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 return temperature sensor, Shorted thermistor, Loose sensor clip causing poor contact, Wiring harness damage
How to Fix / Troubleshooting
Safety: Turn off power and allow the system to cool.
- Locate return thermistor: Typically fitted on the return pipe to the heat exchanger or on the hydraulic block.
- Check mounting: Ensure the sensor is firmly clipped to the pipe or inserted fully into its pocket with good thermal contact. Refit any insulation that was removed.
- Inspect connector: Unplug and replug the sensor connector to clear minor oxidation. Do not pull on the wires; grip the plug body.
- Visual check of harness: Look for pinched or abraded wires where they pass through grommets or near the casing.
- Restart: Restore power and reset. If F5 persists, the sensor likely needs replacement.
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Repair Difficulty
Medium
3/5
Required Part
Return Thermistor
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