A12
Herz
Control Board Internal Fault
Boiler shows A12, will not start, sometimes after power surge
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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
Damaged Herz main PCB, power surge or incorrect polarity, internal relay failure, corrupted firmware
How to Fix / Troubleshooting
Safety: Isolate power before touching any wiring. Electronic repairs should only be done by qualified personnel.
- Power cycle: Turn off the boiler at the isolator for 2–3 minutes, then restore power and see if the error clears.
- Check supply: Verify correct voltage and polarity at the boiler terminals. Ensure neutral and earth are correctly connected.
- Inspect PCB visually: Look for burn marks, swollen capacitors, or loose connectors on the Herz control board.
- If A12 remains: The main PCB likely needs replacement and configuration with Herz service tools. Contact an authorised service partner.
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Repair Difficulty
Professional Required
5/5
Required Part
Herz main control PCB
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