Vaillant F.33 Fault Code: Causes, Fixes & Repair Costs
What does the Vaillant F.33 fault code mean?
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.
General guidance only — not a substitute for professional advice. 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 dirty fan Common
The fan is the most frequent culprit. Over time, dust and debris can coat the fan blades, reducing airflow to the point where the air pressure switch never closes. In other cases the fan motor bearings wear out or moisture ingress damages the motor, causing the fan to run slowly or not at all. With no reliable airflow, the boiler locks out immediately on ignition attempt.
- Defective air pressure switch Common
The air pressure switch — typically located in the top-right area of the boiler, outside the combustion chamber — can fail independently of the fan. The diaphragm inside can split or stick, or the contacts can become corroded, preventing the switch from signalling the PCB that safe airflow is present. An engineer can test the switch quickly with a multimeter to confirm whether it is opening and closing at the correct differential pressure.
- Wiring or connector fault Sometimes
Loose plugs, corroded pins, or damaged cable harnesses between the fan, the air pressure switch, and the PCB can interrupt the signal path and produce an F.33 lockout even when both the fan and switch are mechanically sound. Engineers will inspect the connectors at both ends of the harness as part of a systematic diagnosis.
- Blocked or poorly installed flue Sometimes
A blocked flue outlet or air intake — caused by a bird's nest, debris, or ice in very cold weather — restricts the airflow the fan can generate, preventing the pressure switch from operating correctly. In some cases an incorrectly sized or routed flue installation creates a permanent restriction that triggers the code.
- PCB failure Rare
If the fan and pressure switch both test as healthy and all wiring checks out, the PCB itself may be failing to supply the correct voltage to the fan or to read the pressure switch signal accurately. PCB faults are the least common cause of F.33 and are usually reached by elimination rather than as a first diagnosis.
How to fix it
- Reset the boiler once or twice DIY safe
Press and hold the reset button (the flame symbol with a diagonal line through it on most Vaillant models) for around three seconds until the boiler attempts to restart. If it fires up normally and the F.33 does not return within a day or two, the fault may have been a transient condition. If the lockout comes back, do not keep resetting — repeated resets will not fix an underlying component failure and can mask a developing fault.
- Check your gas supply DIY safe
Make sure your gas meter isolation valve is fully open and that other gas appliances in your home (cooker, gas fire) are working normally. If there is a wider gas supply issue, contact your gas supplier rather than an engineer.
- Inspect the flue terminal for obvious blockages DIY safe
Go outside and look at where the flue exits the wall or roof. Check for any visible obstruction such as leaves, a bird's nest, or ice covering the terminal grille. Do not probe inside the flue, but if you can see a clear blockage at the terminal that you can safely remove without going near gas components, do so. If the terminal appears unobstructed, the issue is most likely internal.
- Do not attempt to open the boiler casing or touch internal components Gas Safe engineer
The fan, air pressure switch, pressure switch hoses, wiring harness, and PCB are all inside the boiler casing. Working on these components without Gas Safe registration is illegal for gas-connected work and potentially dangerous. Leave all internal inspection and testing to a registered engineer.
- Have a Gas Safe engineer test the fan and air pressure switch Gas Safe engineer
A qualified engineer will remove the boiler casing and check whether the fan is spinning freely and producing adequate airflow. They will use a multimeter to verify the air pressure switch opens and closes within specification. A dirty fan can sometimes be cleaned and restored to full function; a failed fan or pressure switch will need to be replaced. The air pressure switch is the cheaper first port of call at roughly £30 for the part.
- Have the engineer inspect wiring, connectors, and — if necessary — the PCB Gas Safe engineer
If the fan and pressure switch are both functioning correctly, the engineer will trace the wiring harness for damaged insulation, corroded pins, or loose connectors at the fan plug and PCB terminals. A PCB fault is diagnosed last, after all other components have been ruled out, given the significantly higher replacement cost.
Parts you may need
- Air pressure switch · from £30
- Boiler fan / flue fan assembly · from £140
- Wiring harness / cable loom · from £45
- PCB (printed circuit board) · from £400
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 £120–£380, depending on the underlying cause.
Frequently asked questions
Can I fix a Vaillant F.33 fault myself?
The only safe DIY actions are attempting one or two resets, checking your gas supply is on, and looking for obvious external flue blockages. All internal work — testing the fan, replacing the air pressure switch, checking wiring, or touching the PCB — must be carried out by a Gas Safe registered engineer. Opening the boiler casing and working on its components without the appropriate registration is both illegal for gas-related work and potentially dangerous.
How much does it cost to repair a Vaillant F.33 fault in the UK?
The most common repairs — replacing the air pressure switch or the fan — typically cost between £120 and £380 all-in once you include parts and labour. An air pressure switch is around £30 for the part and is usually tried first as the cheaper option. A fan replacement adds more, with the part alone around £140 plus an hour or two of labour at typical rates of £50–£80 per hour. If the fault turns out to be a PCB, costs rise considerably — a replacement PCB can be £350–£450 for the part alone — so at that point it may be worth weighing up replacement versus repair, especially on an older boiler.
Why does my Vaillant boiler show F.33 in cold weather?
In very cold conditions, the flue terminal can become partially blocked by ice, or condensate pipes can freeze and cause back-pressure issues that affect airflow sensing. Additionally, a fan that is borderline on performance during milder weather may fail to generate enough airflow when the cold thickens any internal lubricants in the motor. If F.33 appears consistently when temperatures drop, ask your engineer to check both the flue terminal arrangement and the fan's condition.
What is the difference between F.33 on an ecoTEC and on an older Vaillant model?
On ecoTEC models the F.33 description refers to the fan frost protection function activating, whereas on older Turbomax and ThermoCompact models (particularly 837 and 637 variants) the code is described as a failure to receive a switching signal from the air pressure switch circuit. In practice the diagnostic approach is virtually identical for both: an engineer will test the fan, the air pressure switch, and the wiring between them, because both code descriptions point to the same underlying safety system — the fan must run correctly and the pressure switch must confirm adequate airflow before the boiler will fire.