F37 Boiler Fault Code: What It Means on Every Brand

The F37 fault code appears on 4 brands — Ferroli, Johnson & Starley, Protherm 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 F37 means	Severity	DIY?
FFerroli	The F37 code appears when the boiler's water pressure sensor detects that system pressure has dropped to around 0.4 bar or below — too low for the boiler to operate safely. Full guide →	Engineer may be needed	Some checks
JSJohnson & Starley	F37 on a Johnson & Starley unit signals low water pressure in the sealed heating circuit — it applies to the models that carry water: WarmCair D/DW variants providing hot water and central… Full guide →	Engineer may be needed	Some checks
PProthermshown as F.37	The F.37 code appears on Protherm boilers when the pressure-monitoring circuit detects that water pressure in the central heating system has dropped to a dangerously low level — typically… Full guide →	Lockout — engineer likely	Some checks
VVaillantshown as F.37	The F.37 code appears on Vaillant boilers when the PCB detects that the fan is not spinning at the expected speed — or cannot confirm that it is running at all. Full guide →	Lockout — engineer likely	No

F37 brand by brand

Ferroli F37

The F37 code appears when the boiler's water pressure sensor detects that system pressure has dropped to around 0.4 bar or below — too low for the boiler to operate safely. Ferroli classifies F37 as a blocking fault, meaning it will prevent the boiler from firing until the pressure issue is resolved. In many cases the boiler will recover automatically once the correct pressure is restored, without needing a full reset. Normal operating pressure for most Ferroli systems should sit between 1.0 and 1.5 bar when the system is cold, with 1.2 bar being the ideal target.

Causes, fixes & costs for Ferroli F37 → · All Ferroli codes

Johnson & Starley F37

F37 on a Johnson & Starley unit signals low water pressure in the sealed heating circuit — it applies to the models that carry water: WarmCair D/DW variants providing hot water and central heating, and Aquair heat interface units. When system pressure falls below around 0.5 bar the pressure sensor reports it and the water side shuts down, because circulating and heating an under-pressurised system risks pump damage and kettling. The usual causes are the familiar sealed-system suspects: gradual loss through tiny weeps, a recent radiator bleed, a discharging expansion vessel pushing water out of the pressure-relief valve, or an actual leak. Re-pressurising via the filling loop is a normal user task; frequent recurrence is the investigation trigger.

Causes, fixes & costs for Johnson & Starley F37 → · All Johnson & Starley codes

Protherm F.37

The F.37 code appears on Protherm boilers when the pressure-monitoring circuit detects that water pressure in the central heating system has dropped to a dangerously low level — typically around 0.4 bar or below. The boiler locks out and shuts down to protect itself from running dry. On Protherm displays, the letter 'F' flashes alongside '37' to indicate a permanent fault condition. The code can be triggered either by genuinely low system pressure or by a faulty pressure sensor sending incorrect readings even when the actual pressure is adequate. In either case, the boiler will not provide central heating or hot water until the fault is resolved.

Causes, fixes & costs for Protherm F.37 → · All Protherm codes

Vaillant F.37

The F.37 code appears on Vaillant boilers when the PCB detects that the fan is not spinning at the expected speed — or cannot confirm that it is running at all. The fan serves two critical purposes: drawing fresh air into the combustion chamber so gas burns correctly, and expelling waste combustion gases safely through the flue. If the boiler's control board cannot verify that the fan is operating within the correct speed range, it locks the boiler out as a safety measure to prevent dangerous gases accumulating or unsafe combustion taking place. Note that Vaillant uses related codes for fan issues across its range — F.32 is the fan fault code on the ecoTEC Plus, while F.37 and F.38 cover fan speed deviations on other Vaillant models. If you see F.38, the diagnostic approach is essentially the same as for F.37.

Causes, fixes & costs for Vaillant F.37 → · All Vaillant codes

Frequently asked questions

Is the F37 fault code the same on every boiler?

No. F37 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 F37 fault myself?

On some brands there are safe homeowner checks first — typically system pressure, the condensate pipe in cold weather, and a single reset. Where the code is a simple pressure or reset condition, that may be all it needs. Never open the boiler casing yourself.

Who should I call about a F37 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.