F.33 Boiler Fault Code: What It Means on Every Brand

The F.33 fault code appears on 4 boiler brands — Glow-worm, Protherm, Saunier Duval and Vaillant — and on all of them it points to the same kind of problem. The details still differ by brand (thresholds, sub-codes, reset procedure), so use the table below to jump to the page for your boiler.

Lockout — engineer likely 4 brands use this code

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.33 means	Severity	DIY?
GWGlow-wormshown as F33	The F33 code on a Glow Worm boiler signals that the fan has malfunctioned or that there is an airflow problem preventing safe operation. Full guide →	Lockout — engineer likely	No
PProtherm	The F.33 code on a Protherm boiler indicates that the air pressure switch — also called the differential relay or monostat — is failing to confirm to the boiler's control board that the… Full guide →	Lockout — engineer likely	No
SDSaunier Duval	The F.33 fault code on Saunier Duval boilers has two distinct meanings depending on the model. Full guide →	Lockout — engineer likely	No
VVaillant	The F.33 code appears on Vaillant boilers in two closely related scenarios. Full guide →	Lockout — engineer likely	No

F.33 brand by brand

Glow-worm F33

The F33 code on a Glow Worm boiler signals that the fan has malfunctioned or that there is an airflow problem preventing safe operation. When this fault appears, the boiler locks out and will not attempt to fire until the underlying issue is resolved. The fan's job is to push combustion gases safely out through the flue — if it cannot reach the correct operating speed, the boiler's control board cuts power as a safety precaution to prevent potentially harmful gases, including carbon monoxide, from entering your home. On UK models specifically, F33 relates to a fan or airflow fault rather than the air pressure switch definition that appears in some international reference guides. You may also see the closely related F3 code on some Glow Worm models, which is officially described as the fan being unable to reach ignition speed — the cause and remedy are essentially the same.

Causes, fixes & costs for Glow-worm F33 → · All Glow-worm codes

Protherm F.33

The F.33 code on a Protherm boiler indicates that the air pressure switch — also called the differential relay or monostat — is failing to confirm to the boiler's control board that the fan is operating correctly. Specifically, the switch is not returning to its off position when the fan is not running, which tells the boiler something is wrong with how air pressure is being sensed. The pressure switch is a critical safety component: when the fan starts, it creates a draught that causes a small membrane inside the switch to move, closing the electrical contacts and giving the PCB the green light to open the gas valve. If the switch cannot confirm the fan is working — whether due to a fault with the fan itself, a blockage in the flue, a faulty switch, or damaged wiring — the boiler locks out rather than risk flue gases not being expelled safely. This is a safety lockout and the boiler will not fire until the underlying issue is resolved.

Causes, fixes & costs for Protherm F.33 → · All Protherm codes

Saunier Duval F.33

The F.33 fault code on Saunier Duval boilers has two distinct meanings depending on the model. On most condensing combi boilers, it indicates that the air pressure switch contact is stuck in the closed position — meaning the boiler's control board is detecting an incorrect signal from the switch even when there is insufficient airflow. This triggers a safety lockout to prevent unsafe operation and potential combustion gas issues. On certain Isofast models (including the Isofast F 35 E-A, F 35 E H-MOD, and Isofast Condens F35), F.33 instead signals a radio communication failure between the boiler's control panel and a wireless room thermostat (TA1). Check your boiler's manual or the display context to confirm which interpretation applies to your unit.

Causes, fixes & costs for Saunier Duval F.33 → · All Saunier Duval codes

Vaillant F.33

The F.33 code appears on Vaillant boilers in two closely related scenarios. On ecoTEC models (including the popular ecoTEC Plus range), it signals that the boiler's fan frost protection mode has activated — the control board has detected conditions that risk freezing damage to the heat exchanger and has shut the boiler down as a precaution. On older Vaillant models such as the Turbomax and ThermoCompact (including 837 and 637 variants), F.33 points specifically to the boiler receiving no switching signal from the air pressure switch circuit, again typically caused by a fan or pressure switch problem. In both cases the boiler locks out and stops producing heat or hot water until the underlying fault is resolved. The common thread across all affected models is the fan-and-air-pressure-switch safety system: the fan must spin and generate sufficient airflow to close the air pressure switch before the gas valve is permitted to open. If the fan runs poorly, the pressure switch fails to respond, or the flue is obstructed, F.33 is the result.

Causes, fixes & costs for Vaillant F.33 → · All Vaillant codes

Frequently asked questions

Is the F.33 fault code the same on every boiler?

Broadly, yes — on Glow-worm, Protherm, Saunier Duval and Vaillant the F.33 code points to the same class of fault, and the recommended response is similar. Model-specific details such as sub-codes and reset procedures still vary, so follow the page for your own brand.

Can I fix a F.33 fault myself?

Not usually. On every brand listed, F.33 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.33 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.