Ferroli F39 Fault Code: Causes, Fixes & Repair Costs
What does the Ferroli F39 fault code mean?
The F39 code on Ferroli boilers has two distinct meanings depending on the specific model. On most Ferroli boilers, F39 indicates a fault with the optional external (outdoor) temperature probe — the sensor used for weather compensation has either failed, developed a short circuit in its wiring, or become disconnected. On certain Ferroli models such as the DOMIcondens HE and Optimax HE Plus, F39 instead signals that the heat exchanger protection function has been triggered, typically due to poor circulation, trapped air in the system, or a faulty temperature sensor. Check your boiler's manual to confirm which meaning applies to your model before taking any action.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Faulty or degraded outdoor temperature probe Common
The external probe used for weather compensation gradually degrades over time, especially when exposed to the elements. If the sensor's resistance drifts outside the expected range, the boiler control board logs F39. This is the most common trigger on models where F39 denotes an external probe fault.
- Damaged or shorted probe wiring Common
The cable running from the outdoor probe back into the boiler can be damaged by weathering, rodents, or accidental impact. A short circuit or broken conductor in this wiring causes the boiler to read an invalid signal and raise F39.
- Probe disconnected or not fitted after enabling weather compensation Sometimes
If the outdoor temperature compensation feature has been activated in the boiler's settings but no probe is physically connected — or the probe has come loose from its terminal — the boiler detects an open circuit and triggers F39.
- Poor system circulation triggering heat exchanger protection Sometimes
On models where F39 means heat exchanger protection, restricted water flow through the system — caused by closed radiator valves, a failing pump, or a blocked filter — can cause the heat exchanger to overheat and activate the protection fault.
- Air trapped in the system Sometimes
On heat exchanger protection models, air locks in the system reduce effective water circulation, causing localised overheating at the heat exchanger and triggering F39.
- Faulty flow or return temperature sensor Rare
A damaged NTC thermistor on the flow or return pipe can send incorrect temperature readings to the PCB, causing it to believe the heat exchanger is in danger and raise the F39 protection fault.
How to fix it
- Identify which F39 meaning applies to your boiler DIY safe
Look up your specific Ferroli model in the boiler manual or on the rating plate inside the casing. Confirm whether F39 on your model refers to an external probe fault or heat exchanger protection activation. This determines which checks are relevant.
- Reset the boiler once or twice DIY safe
Press and hold the reset button (usually marked 'R' on the control panel) for one to two seconds. Allow the boiler to restart fully before checking whether the fault clears. If F39 returns after two resets, do not keep resetting — a persistent code indicates a genuine fault that requires investigation.
- Check system pressure (heat exchanger protection models) DIY safe
Look at the pressure gauge on the boiler front. It should read between 1.0 and 1.5 bar when the system is cold. If it is below 1.0 bar, top it up using the filling loop until it reaches around 1.2 bar, then attempt a reset. If the pressure is already correct, move to the next step.
- Bleed radiators to remove trapped air (heat exchanger protection models) DIY safe
If F39 relates to heat exchanger protection on your model, turn off the boiler and allow it to cool. Use a radiator bleed key to release any trapped air from radiators, starting on the ground floor and working upward. Re-check system pressure afterwards and top up if needed.
- Check that all radiator valves are open (heat exchanger protection models) DIY safe
Walk around the property and confirm that thermostatic radiator valves and lockshield valves are open on all radiators. A system with too many closed radiators can reduce flow enough to trigger heat exchanger protection.
- Visually inspect the outdoor probe and its cable DIY safe
If your model has an external weather compensation probe, locate it on the outside wall of the property. Check whether the probe housing is cracked or clearly damaged and whether the cable appears intact all the way to where it enters the building. Do not open the boiler casing or touch the wiring inside.
- Have a Gas Safe engineer inspect and repair the fault Gas Safe engineer
Any work beyond the checks above — including testing and replacing the outdoor probe, inspecting internal wiring connections, replacing a temperature sensor, checking the pump, or diagnosing a PCB issue — must be carried out by a Gas Safe registered engineer. Provide them with the F39 code and your boiler model so they can bring the appropriate parts.
Parts you may need
- External/outdoor temperature probe (NTC sensor) · from £25
- Flow or return NTC thermistor · from £20
- Probe wiring harness · from £18
- Central heating pump · from £65
- PCB (printed circuit board) · from £160
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £90–£200, depending on the underlying cause.
Frequently asked questions
My Ferroli shows F39 but I don't have an outdoor probe fitted — why?
If weather compensation mode has been switched on in the boiler's settings but no outdoor probe is physically connected, the boiler will raise F39 because it expects a probe signal and receives none. A Gas Safe engineer can either connect a probe or disable the weather compensation setting so the boiler no longer looks for one.
Can I fix the F39 fault myself?
You can safely try a reset, check system pressure, bleed radiators, and visually inspect the outdoor probe cable for obvious damage. However, replacing the probe, testing wiring continuity inside the boiler, or diagnosing a circulation problem all require a Gas Safe registered engineer. Do not remove the boiler casing yourself.
How much does it cost to fix a Ferroli F39 fault in the UK?
For most people, the repair involves replacing the outdoor probe or a temperature sensor, which typically costs £90–£200 including parts and a Gas Safe engineer's labour. If the fault turns out to be a faulty PCB the cost rises considerably — PCB replacement on Ferroli boilers is usually £200–£300 all in — but this is less common. Always get a diagnosis before authorising major parts replacements.
Should I keep resetting the boiler if F39 keeps coming back?
No. One or two resets to see whether the fault clears is reasonable, but if F39 returns repeatedly it means the underlying problem has not been resolved. Continuous resetting can mask a worsening fault and may damage components further. Book an engineer visit if the code reappears after two resets.