F39 Low

Ferroli F39 Fault Code

The Ferroli F39 fault code means: your boiler is having trouble communicating with the outdoor weather sensor, which means it cannot automatically adjust its temperature based on outside conditions.. It appears on 14 Ferroli models in our database.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description: External probe fault

What causes the Ferroli F39 fault?

This fault occurs when the boiler's internal components detect a rapid, unsafe rise in temperature that cannot be dissipated. It is usually caused by a blockage in the pipework, a buildup of air, or a pump that has failed and stopped moving water through the system. In some cases, the temperature sensor itself may be faulty and providing an incorrect reading to the control board.

This issue often surfaces when the heating is first turned on after summer, as circulation pumps can seize up during long periods of inactivity.

Typical repair cost £120-200 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Ferroli models that show the F39 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Ferroli F39 fault code information on this page is sourced and cross-checked against the official Ferroli service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026