E12
Alpha
Primary Heat Exchanger Sensor / Calibration Fault
Boiler displays E12 and locks out; may be associated with incorrect temperature readings or rapid cycling.
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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
Misfitted primary NTC sensor, Scale or sludge in primary heat exchanger, Faulty primary NTC, PCB calibration error
How to Fix / Troubleshooting
Safety: Turn off power and allow the boiler to cool before working on the heat exchanger.
- Check sensor seating: Ensure the primary NTC sensor is fully inserted into its pocket on the heat exchanger and secured with the correct clip.
- Inspect for leaks: Look for any signs of water staining or corrosion around the heat exchanger and sensor pocket.
- System cleanliness: If radiators are cold at the bottom or water is dirty, the system may be sludged. A professional powerflush and installation of an Alpha magnetic filter may be required.
- Sensor test: A qualified engineer can test the NTC resistance and replace the sensor if faulty.
PCB calibration or replacement must be carried out by an Alpha-approved technician.
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Repair Difficulty
Hard
4/5
Required Part
Primary Heat Exchanger NTC Sensor
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