F.42 Boiler Fault Code: What It Means on Every Brand

The F.42 fault code appears on 4 brands — Ferroli, Protherm, Saunier Duval and Vaillant — 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 4 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 F.42 means	Severity	DIY?
FFerrolishown as F42	The F42 code appears when the Ferroli boiler's control board detects an abnormal or implausible reading from one or more of its NTC thermistors — specifically the heating flow sensor or… Full guide →	Lockout — engineer likely	No
PProtherm	The F.42 code appears on Protherm boilers when the control board detects that the coding resistor (R1) is returning a value outside the expected range. Full guide →	Lockout — engineer likely	No
SDSaunier Duval	The F.42 code (sometimes displayed as F42, with the decimal point appearing between flashing digits on the display) signals one of two related electronic faults depending on your Saunier… Full guide →	Engineer may be needed	No
VVaillant	The F.42 code appears when your Vaillant boiler cannot correctly read its coding resistor — a small component (sometimes a plug or inline connector) that identifies the boiler model… Full guide →	Lockout — engineer likely	No

F.42 brand by brand

Ferroli F42

The F42 code appears when the Ferroli boiler's control board detects an abnormal or implausible reading from one or more of its NTC thermistors — specifically the heating flow sensor or the central heating return sensor. On many Ferroli models the fault triggers when the difference between the flow and return sensor readings exceeds around 12°C, suggesting at least one sensor is producing an inaccurate value. Because these sensors feed directly into the boiler's safety protection logic, the board locks the boiler out rather than risk overheating. You may also see this displayed as A42 on certain Ferroli models — both relate to the same underlying sensor protection fault.

Causes, fixes & costs for Ferroli F42 → · All Ferroli codes

Protherm F.42

The F.42 code appears on Protherm boilers when the control board detects that the coding resistor (R1) is returning a value outside the expected range. The coding resistor is a small component on the PCB that effectively tells the boiler's electronics what type of appliance it is and how it should behave. When this value doesn't match what the board expects — whether because the resistor has failed, the wrong product code was programmed in at installation, or the PCB itself is damaged — the boiler locks out and displays F.42 as a safety precaution. This is a board-level electronic fault and sits in the more serious category of Protherm error codes (F.20–F.42), meaning it will not clear on its own without the underlying issue being addressed.

Causes, fixes & costs for Protherm F.42 → · All Protherm codes

Saunier Duval F.42

The F.42 code (sometimes displayed as F42, with the decimal point appearing between flashing digits on the display) signals one of two related electronic faults depending on your Saunier Duval model. On most models it indicates a heating sensor fault — specifically a problem with an NTC thermistor that monitors flow or return water temperature. On newer condensing models such as the Thema Condens F 25 it can instead point to a coding resistor or eBus communication fault, meaning the main PCB cannot correctly identify the boiler's configuration or has lost reliable communication with a connected controller. In practice both interpretations sit within the same family of electronic and sensor-circuit faults, and a Gas Safe engineer will quickly establish which applies to your specific unit.

Causes, fixes & costs for Saunier Duval F.42 → · All Saunier Duval codes

Vaillant F.42

The F.42 code appears when your Vaillant boiler cannot correctly read its coding resistor — a small component (sometimes a plug or inline connector) that identifies the boiler model variant and the type of gas it runs on to the PCB. When the PCB detects a short circuit, an open circuit, or a mismatch between the physical resistor and the gas-type setting stored in diagnostic parameter D.087, it locks the boiler out as a safety measure. In plain terms, the boiler has lost confidence in its own identity and refuses to fire until the issue is resolved. This is entirely an engineer-level fault; there are no meaningful homeowner checks that will clear it.

Causes, fixes & costs for Vaillant F.42 → · All Vaillant codes

Frequently asked questions

Is the F.42 fault code the same on every boiler?

No. F.42 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 F.42 fault myself?

Not usually. On every brand listed, F.42 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 F.42 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.