F39 Boiler Fault Code: What It Means on Every Brand

The F39 fault code appears on 2 brands — Ferroli and Protherm — and it does not mean the same thing on each. The same two or three characters can signal a pressure problem on one boiler and an ignition fault on another, so identify your brand first and use the table below to jump to the right page.

Up to: Lockout — engineer likely 2 brands use this code Meaning differs by brand

Which brand is your boiler?

The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.

Brand	What F39 means	Severity	DIY?
FFerroli	The F39 code on Ferroli boilers has two distinct meanings depending on the specific model. Full guide →	Engineer may be needed	No
PProthermshown as F.39	The F.39 fault code indicates that the Protherm boiler's electronic control board (PCB) has been unable to correctly identify or communicate with the boiler's own configuration hardware… Full guide →	Lockout — engineer likely	No

F39 brand by brand

Ferroli F39

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.

Causes, fixes & costs for Ferroli F39 → · All Ferroli codes

Protherm F.39

The F.39 fault code indicates that the Protherm boiler's electronic control board (PCB) has been unable to correctly identify or communicate with the boiler's own configuration hardware during its internal self-check. In plain terms, the boiler starts up, runs through a diagnostic routine to confirm what components and settings it is working with, and something in that process has gone wrong — either the configuration data doesn't match what the PCB expects, or a component involved in that handshake isn't responding properly. F.39 sits within the F.20–F.42 fault block on Protherm boilers, a group of codes associated with serious internal control and safety malfunctions. Closely related codes include F.41 (gas regulation error) and F.42 (coding resistor fault), which can help an engineer narrow down whether the root issue is a configuration mismatch, a wiring fault, or PCB failure itself.

Causes, fixes & costs for Protherm F.39 → · All Protherm codes

Frequently asked questions

Is the F39 fault code the same on every boiler?

No. F39 means different things on different brands — see the table above. Always identify your boiler's brand (it is printed on the front casing) before acting on any advice about the code.

Can I fix a F39 fault myself?

Not usually. On every brand listed, F39 points to a fault that needs a suitably registered engineer to diagnose and repair. A single reset to rule out a transient glitch is reasonable; repeated resets are not.

Who should I call about a F39 fault?

For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. If you smell gas at any point, leave the property and call the National Gas Emergency line on 0800 111 999.

General guidance only — not a substitute for professional advice, and not certified by a registered engineer. Any work on a gas appliance must be carried out by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.