F5
ATAG
Flow Switch / Flow Sensor Fault
Furnace displays F5, heating output limited or no heating
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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
Defective flow sensor, Blocked system filter or strainer, Air lock in system preventing flow
How to Fix / Troubleshooting
Safety first: Turn off power and allow the system to cool before opening any water components.
- Check system pressure and top up if low.
- Bleed radiators and high points to remove air.
- Clean any inline system filter or strainer on the return to the boiler.
- Locate the flow sensor and check its connector and wiring.
- If the sensor is removable, inspect for debris and clean gently.
- If F5 persists, the flow sensor may need replacement by a professional.
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Repair Difficulty
Medium
3/5
Required Part
ATAG flow sensor
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