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F39

Ferroli F39 on a gas boiler: outdoor temperature sensor fault

Verified with sources2 sourcesReviewed Oct 4, 2026
Technician coming? See the 3 steps they must show you
Wall-mounted combi gas boiler installed in a home
Kai KemmannCC BY-SA 4.0Image source

Risk level

Low

Usually safe to check without specialist tools.

Urgency

Low

Quick answer

Ferroli F39 is an outdoor sensor fault: the sensor that reads the outside temperature for weather compensation. Check from the ground whether the sensor on the wall is loose or damaged and tell the technician who installed it.

Safety first

Stop and check this first

  • If you smell gas: do not switch lights or electrical appliances on or off, do not use anything that can spark and do not try to reset the boiler. Open the windows, leave the home and call your gas distributor's emergency number from outside.
  • Do not pull or splice the outdoor sensor cable yourself: a short circuit in that cable is one of the causes of F39 according to Ferroli.

Safe checks you can perform

  • Outdoor sensor symbol on the display
  • Sensor seen from outside
  • Recent settings changes
  • Inspection by the technician

Leave these tasks to a qualified professional

  • Do not attempt this professional task: Measure the outdoor sensor
  • Do not attempt this professional task: Wiring check
  • Do not attempt this professional task: Weather compensation settings

What a technician must be able to justify

Technician visit

These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.

These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.

  1. Measure the outdoor sensor

    What they doThe technician measures the sensor resistance with a multimeter and compares it with the manufacturer's table at the current outside temperature.

    They must show youthe value measured against the manufacturer's specification.

    In writingif the sensor was replaced, its part number and why.

  2. Wiring check

    What they doThe technician checks continuity of the cable between the sensor and the boiler and that it is not shorted.

    They must show youthe continuity test.

    In writingthe damaged section and the repair.

  3. Weather compensation settings

    What they doThe technician checks whether weather compensation is on and whether it matches the sensor fitted.

    They must show youthe curve set.

    In writingthe final setting.

Before accepting a part or repair

Nothing has been shown yet. Do not accept a part swap based on the code alone.

Follow these steps

Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.

  1. Outdoor sensor symbol on the display

    No special tools

    Who:
    you.
    How:
    on the RRT 28 C, look for the outdoor sensor detected symbol, which Ferroli shows when an outdoor sensor is present.
    Result that matters:
    whether the boiler sees the sensor.
    If it is missing:
    the boiler does not detect the sensor; tell the technician.
    If it shows:
    the sensor is detected and the fault may be in the reading or the cable.
    Evidence to keep:
    photo of the display showing F39.

    Evidence: Ferroli BLUEHELIX TECH RRT 28 C instrucciones de uso instalación y mantenimiento

  2. Sensor seen from outside

    No special tools

    Who:
    you.
    How:
    find the outdoor sensor on the outside wall and look at it from the ground or a window, without touching the cable or climbing.
    Result that matters:
    whether the sensor is in place with its cable intact.
    If it is loose, broken or the cable is cut:
    it fits a disconnected or faulty sensor; tell the technician.
    If it looks fine:
    the fault may be in hidden wiring or the settings.
    Evidence to keep:
    photos of the sensor and the visible cable.

    Evidence: Ferroli BlueHelix HiTech RRT C instrucciones de uso instalación y mantenimiento · Ferroli BLUEHELIX TECH RRT 28 C instrucciones de uso instalación y mantenimiento

  3. Recent settings changes

    No special tools

    Who:
    you.
    How:
    think back on whether anyone changed the weather compensation curve or removed the sensor during work on the wall. On the RRT 28 C that menu opens by holding Reset for 5 seconds and curve 0 switches the function off.
    Result that matters:
    whether F39 started after a change.
    If it matches a change:
    tell the technician; Ferroli's fix is to connect the sensor or switch weather compensation off.
    If nothing changed:
    it points to a faulty sensor or wiring.
    Evidence to keep:
    a note of the date of the change.

    Evidence: Ferroli BlueHelix HiTech RRT C instrucciones de uso instalación y mantenimiento · Ferroli BLUEHELIX TECH RRT 28 C instrucciones de uso instalación y mantenimiento

  4. Inspection by the technician

    Tools required

    Who:
    an authorised Ferroli technician or installer.
    How:
    checks the outdoor sensor, its wiring and the boiler weather compensation settings.
    Result that matters:
    whether the sensor reads correctly and the settings match what is installed.
    If the sensor or cable is faulty:
    the cable is repaired or the sensor replaced.
    If no sensor is fitted:
    one is connected or weather compensation is switched off.
    Evidence to keep:
    job sheet with what was measured and the final setting.

    On the HiTech the curves are set from the password protected installer menu. Opening the boiler or touching the wiring is technician work.

    Evidence: Ferroli BlueHelix HiTech RRT C instrucciones de uso instalación y mantenimiento · Ferroli BLUEHELIX TECH RRT 28 C instrucciones de uso instalación y mantenimiento

Still not solved

If the error persists, stop here. See what the technician must do and show you.

Where this code applies

A code is meaningful only inside the right product and version context.

Primary scope

System
Boilers
Brand
Ferroli
Product type
Gas boiler

Known code variants

  • BlueHelix F39
  • Error F39 Ferroli
  • F 39
  • F39 sonda exterior
  • Ferroli F 39

What it means

On the BlueHelix Tech RRT 28 C and BlueHelix HiTech RRT C, F39 is an outdoor sensor fault. The sensor is optional and enables weather compensation (adaptive temperature): the boiler lowers the flow temperature as the outside temperature rises. Ferroli gives two causes: a faulty sensor or shorted wiring, or the sensor disconnected after weather compensation was switched on. F39 is a temporary lockout. The code does not say whether the sensor, its cable or the settings are at fault.

Warnings and stop conditions

  • Criticaleditorial warning

    If you smell gas: do not switch lights or electrical appliances on or off, do not use anything that can spark and do not try to reset the boiler. Open the windows, leave the home and call your gas distributor's emergency number from outside.

    Evidence: Ferroli BlueHelix HiTech RRT C instrucciones de uso instalación y mantenimiento · Ferroli BLUEHELIX TECH RRT 28 C instrucciones de uso instalación y mantenimiento

  • Loweditorial warning

    Do not pull or splice the outdoor sensor cable yourself: a short circuit in that cable is one of the causes of F39 according to Ferroli.

    Evidence: Ferroli BlueHelix HiTech RRT C instrucciones de uso instalación y mantenimiento · Ferroli BLUEHELIX TECH RRT 28 C instrucciones de uso instalación y mantenimiento

Symptoms and causes

Probable causes · in order

  • Faulty sensor or wiringPossible

    The outdoor sensor is faulty or its cable has a short circuit. It fits if the sensor is fitted and connected and F39 still appears.

    Evidence: Ferroli BlueHelix HiTech RRT C instrucciones de uso instalación y mantenimiento · Ferroli BLUEHELIX TECH RRT 28 C instrucciones de uso instalación y mantenimiento

  • Weather compensation on without a sensorPossible

    Weather compensation was switched on but the sensor is not connected. It fits after a settings change or if the sensor was removed from the wall.

    Evidence: Ferroli BlueHelix HiTech RRT C instrucciones de uso instalación y mantenimiento · Ferroli BLUEHELIX TECH RRT 28 C instrucciones de uso instalación y mantenimiento

Virtual technician

Does F39 still appear after these checks?

Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.

Start diagnosis

Sources and technical references

  1. 1

    Ferroli BlueHelix HiTech RRT C instrucciones de uso instalación y mantenimiento

    Ferroli · 2021 · manual

  2. 2

    Ferroli BLUEHELIX TECH RRT 28 C instrucciones de uso instalación y mantenimiento

    Ferroli · 2018 · manual

Sources: Ferroli BlueHelix HiTech RRT C instrucciones de uso instalación y mantenimiento · Ferroli BLUEHELIX TECH RRT 28 C instrucciones de uso instalación y mantenimientoreviewed 2026-10-04 · verified

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