A26
Herz
Sensor Calibration / Mismatch Fault
Boiler shows A26, system temperature sensor mismatch, erratic operation
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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
Incorrect combination of Herz NTC sensors, wrong sensor type fitted, PCB configured for different sensor curve, wiring cross-connection between sensors
How to Fix / Troubleshooting
Safety: Isolate power before working on sensors and wiring.
- Verify sensor types: Check that all replaced sensors are genuine Herz parts with correct specifications.
- Check wiring order: Ensure flow, return, and DHW sensors are connected to the correct terminals on the PCB.
- Reset parameters: In the installer menu, confirm that the boiler is set for the correct sensor configuration as per Herz manual.
- Replace mismatched sensors: If non-original sensors were used, replace them with the correct Herz NTC sensors.
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Repair Difficulty
Medium
3/5
Required Part
Herz NTC sensor set
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