A9
Junkers
Flow temperature sensor (NTC) fault
Boiler shows A9; heating flow temperature reading is implausible or boiler overheats quickly
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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 NTC sensor, Loose sensor clip or poor contact with pipe, Wiring fault between NTC and PCB, Air lock or no circulation causing local overheating
How to Fix / Troubleshooting
Safety: Allow the boiler to cool before touching any pipework. Isolate power before removing covers.
- Check system pressure: Ensure pressure is within the recommended range (around 1–1.5 bar cold). Top up via the filling loop if needed.
- Bleed air: Bleed radiators and any automatic air vents near the boiler to remove trapped air that can cause hot spots.
- Inspect flow NTC:
- Locate the flow NTC on the flow pipe leaving the heat exchanger.
- Ensure it is firmly clipped to the pipe and that thermal paste (if used) is intact.
- Check the connector for corrosion or loose fit; clean and reseat.
- Measure resistance and compare with Junkers NTC chart; replace if out of tolerance.
- Check pump operation: Confirm that the circulation pump runs when there is a heat demand; listen for operation and feel for vibration.
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Repair Difficulty
Medium
3/5
Required Part
Flow NTC Temperature Sensor
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