F4
Ideal
Primary Flow Thermistor Fault
Display shows F4 and the heater will not run; may occur after overheating or poor circulation.
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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-circuit flow temperature sensor, Shorted thermistor, Damaged sensor wiring, Poor sensor connection at control board
How to Fix / Troubleshooting
Safety: Isolate power. Allow the water heater to cool fully before touching any pipework or sensors.
- Locate flow thermistor: On Ideal water heaters, the primary flow thermistor is usually clipped or screwed into the primary flow pipe or heat exchanger outlet.
- Check connection: Ensure the thermistor plug is fully seated on the sensor and at the control board. Look for corrosion or water ingress in the connector.
- Inspect cable: Follow the cable from the sensor to the board, checking for cuts, crushed sections, or signs of overheating.
- Reset and test: After reseating connectors, restore power and reset the unit. If F4 reappears, the thermistor is likely faulty and should be replaced by a competent person.
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Repair Difficulty
Medium
3/5
Required Part
Primary Flow Thermistor
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