Saunier Duval F.33 Fault Code: Causes, Fixes & Repair Costs
What does the Saunier Duval F.33 fault code mean?
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.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. 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 worn air pressure switch diaphragm Common
The internal diaphragm inside the air pressure switch can deteriorate, perforate, or become clogged over time. When this happens, the switch contact remains stuck in the closed position regardless of actual airflow, sending a false signal to the PCB and triggering lockout. This is the most frequently diagnosed root cause of F.33 on condensing Saunier Duval boilers.
- Blocked or obstructed flue terminal Common
If the external flue outlet is partially or fully blocked — by a bird nest, accumulated debris, ice in cold weather, or vegetation — back-pressure can keep the air pressure switch contact closed when it should not be. Even with a working fan and intact switch, a restricted flue can generate this fault.
- Corroded or loose electrical connections to the pressure switch Sometimes
Oxidised terminals, a loose connector, or degraded cable insulation can create a permanent false contact at the pressure switch wiring. The PCB interprets this as the switch being stuck closed and locks the boiler out, even though the switch itself may be mechanically sound.
- Dirty or failing combustion fan Sometimes
A fan clogged with dust or debris loses efficiency and fails to generate the correct pressure differential across the pressure switch. If the fan is struggling or starting to fail, the pressure switch may not operate correctly, leading to an F.33 lockout.
- Radio communication failure with wireless thermostat (Isofast models only) Sometimes
On Isofast variants, F.33 specifically flags a loss of radio signal between the boiler control panel and the TA1 wireless room thermostat. This can be caused by flat thermostat batteries, an addressing mismatch after a power cycle, or physical obstructions and interference disrupting the radio signal path.
How to fix it
- Check the external flue terminal for blockages DIY safe
Go outside and visually inspect the flue outlet on the external wall or roof. Look for bird nests, leaves, ice, or any physical obstruction covering or partially blocking the terminal. If you can safely clear the obstruction without accessing the boiler's internal components, do so. Do not attempt to dismantle any flue pipework.
- Attempt a boiler reset DIY safe
Press and hold the reset button on your boiler's control panel as described in your user manual. Wait for the boiler to attempt a restart. If it fires successfully and runs without returning F.33, monitor it over the next hour. Do not reset more than two or three times — repeated lockouts indicate a genuine fault that needs professional diagnosis.
- Check thermostat batteries and re-pair if on an Isofast model DIY safe
If your boiler is an Isofast model and you use a wireless room thermostat, replace the batteries in the thermostat with fresh ones. Then follow the pairing or re-addressing procedure in your thermostat's user guide to re-establish the radio link with the boiler. Once paired, attempt a reset.
- Verify your gas supply is active DIY safe
Check that other gas appliances in your home (such as a gas hob) are working normally. If there is no gas elsewhere, contact your gas supplier. If gas is available to other appliances, the supply to the boiler is likely not the issue here.
- Have a Gas Safe engineer inspect and clean the pressure switch hoses and connections Gas Safe engineer
A registered engineer will remove the pressure switch hoses, check them for blockages or splits, and clean the metal connection ports. They will also disconnect the electrical connectors at the switch with the power isolated, inspect each terminal for oxidation, corrosion, or burn marks, clean any affected contacts, and ensure all connectors are firmly re-seated.
- Have a Gas Safe engineer test and replace the air pressure switch if needed Gas Safe engineer
If cleaning the hoses and connections does not resolve the fault, the engineer will test the switch electrically and mechanically. A faulty diaphragm or internally failed switch cannot be repaired — it must be replaced with the correct Saunier Duval-compatible part. This is the most common repair outcome for F.33 on condensing models.
- Have a Gas Safe engineer inspect and service the combustion fan Gas Safe engineer
If the pressure switch is found to be intact, the engineer will inspect the fan for dirt build-up, blade damage, or bearing wear. A clogged or failing fan is cleaned or replaced as required before the boiler is retested.
- Contact a Gas Safe registered engineer if the fault persists or you cannot identify the cause Gas Safe engineer
If the boiler continues to lock out with F.33 after your own basic checks, do not keep resetting it. Book a Gas Safe registered engineer to carry out a full diagnostic inspection. Running a boiler with a pressure switch fault risks unsafe flue gas handling, so prompt professional attention is important.
Parts you may need
- Air pressure switch · from £55
- Pressure switch hose/tubing kit · from £18
- Combustion fan assembly · from £110
- Wireless thermostat (Isofast TA1 compatible) · from £65
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–£300, depending on the underlying cause.
Frequently asked questions
Is the Saunier Duval F.33 code dangerous?
On condensing boiler models, yes — it should be taken seriously. The fault means the boiler has detected that its air pressure switch is stuck in the closed position, which relates to how combustion gases are safely exhausted. The boiler locks itself out as a precaution, which is the correct response. Do not repeatedly reset it and leave it running without getting the fault properly diagnosed, as doing so could risk unsafe flue gas handling. Call a Gas Safe engineer promptly.
Can I fix the F.33 fault myself?
The only checks a homeowner can safely carry out are: inspecting the external flue terminal for obvious blockages, attempting a reset (no more than two or three times), and — on Isofast models with a wireless thermostat — replacing the thermostat batteries and re-pairing the device. All internal work, including inspecting or replacing the air pressure switch, hoses, fan, or electrical connections inside the boiler, must be carried out by a Gas Safe registered engineer. It is both unsafe and illegal for an unregistered person to work on these components.
How much does it cost to repair a Saunier Duval F.33 fault in the UK?
For the most common repair — replacing the air pressure switch — expect to pay roughly £120 to £250 in total, covering the part (around £40–£100 depending on the specific model) and labour (typically £75–£150). If the fault turns out to be a blocked or dirty flue or fan, an engineer may be able to resolve it during the same visit without additional parts costs. Fan replacement is more involved and can push the total toward £250–£300. If the fault relates to a more significant component such as the PCB, costs can be higher — your engineer will advise after diagnosis.
Why does my Saunier Duval show F.33 but the fan seems to be running?
The fan running does not necessarily mean it is producing the correct airflow. A fan that is dirty, has worn bearings, or has a damaged blade may spin but generate insufficient pressure differential to operate the air pressure switch correctly. Equally, the switch itself could have a failed diaphragm that keeps the contact stuck closed regardless of what the fan is doing. A Gas Safe engineer can test both components individually to pinpoint which is at fault.