F21
Ferroli Return NTC Short Circuit

Boiler shows F21; may indicate a return NTC short circuit on some Ferroli boilers.

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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

Shorted return NTC, Moisture in connector, Damaged wiring, PCB fault

How to Fix / Troubleshooting

Safety first: Turn off power before working on the boiler.

  • Engineer checks:
    • Measure resistance of the return NTC; replace if near zero ohms.
    • Inspect connector and wiring for moisture or damage.
    • Check for shorts to earth or other conductors.
    • Replace PCB if sensor and wiring are confirmed good.
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Repair Difficulty

Hard 4/5

Required Part

Return NTC Temperature Sensor
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