A9
Junkers
Return Temperature Sensor Fault / Sensor Mismatch
Display shows A9; water temperature fluctuates or unit cycles rapidly.
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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
Faulty return NTC sensor, Incorrect sensor connection (sensors swapped), Damaged wiring harness, Control board reading error
How to Fix / Troubleshooting
Safety: Disconnect power before working inside the heater.
- Identify sensors: On combination Junkers units, the flow and return NTCs are often similar. Verify each sensor is connected to the correct plug on the PCB according to the wiring diagram.
- Inspect wiring: Follow the cable from the return NTC to the board. Look for pinched or burned sections, especially near hot components.
- Clean connectors: Unplug and replug the sensor connector to remove oxidation. Do not bend pins.
- Test sensor: If you have a multimeter and the resistance chart, compare the return NTC reading to the flow NTC at the same temperature. Large differences indicate a faulty sensor.
- Replace sensor: If defective, install a genuine Junkers return NTC sensor and secure it firmly to the pipe or pocket.
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Repair Difficulty
Medium
3/5
Required Part
Return Temperature NTC Sensor
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