Boiler shows A08, temperature reading is fixed or unrealistic, boiler may cycle rapidly

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.

Possible Causes

Failed Herz return NTC sensor, damaged harness, connector oxidation on return sensor plug, moisture in sensor pocket

Safety: Turn off power and allow the boiler to cool.

Locate return NTC: Find the sensor on the return pipe entering the Herz boiler. Inspect for signs of leakage or corrosion.
Check cable: Follow the cable back to the PCB, ensuring it is not pinched or cut.
Clean and reseat: Disconnect the plug, clean with electrical contact cleaner, and reconnect firmly.
Replace if needed: If the error persists, replace the Herz return NTC sensor following the service manual, then refill and bleed the system if removal requires draining.

Medium 3/5

Herz return NTC sensor

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