Protherm F.33 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.33 fault code mean?
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.
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
- Blocked or obstructed flue Common
A partial or complete blockage in the flue or chimney — caused by debris, a bird's nest, ice, or a section of flue that has come apart — can prevent the fan from creating the draught needed to close the pressure switch. This is one of the more straightforward causes and may be visible from outside the property.
- Faulty or failing fan Common
If the boiler fan has seized, is running too slowly, or has failed entirely, it cannot generate enough air pressure to activate the pressure switch membrane. The boiler will detect this and lock out. Fan failure is a very common cause of F.33 and typically requires replacement by a Gas Safe engineer.
- Faulty or stuck air pressure switch Common
The differential relay itself can fail over time. The membrane inside may degrade, the switch contacts may stick either open or closed, or moisture ingress may cause an intermittent or permanent fault. A stuck-open switch will always signal a fault; a stuck-closed switch may cause a different but related issue. Either way, the PCB receives an incorrect signal.
- Damaged or blocked pressure switch tubes Sometimes
Short rubber or plastic hoses connect the fan housing to the pressure switch. If these tubes split, become kinked, or are blocked by condensation or debris, the switch cannot sense the correct pressure differential — even if the fan is working perfectly. Older boilers are particularly prone to this as the tubing degrades with age.
- Degraded wiring or loose connections Sometimes
Corroded terminals, loose plugs, or damaged wiring in the circuit between the fan, the pressure switch, and the PCB can produce intermittent or permanent F.33 faults. Moisture ingress into connectors is a known issue on older Protherm appliances.
- Faulty PCB Rare
If the control board has failed or developed a fault, it may misinterpret — or fail to receive — the signal from the pressure switch, triggering a lockout even when the fan and switch are both working correctly. PCB faults are relatively rare but should be considered after all other components have been checked.
How to fix it
- Check for an obvious flue blockage DIY safe
Look at the external flue terminal from outside the property. Check for visible obstructions such as debris, ice, bird nests, or a damaged flue cap. If the flue terminal is accessible and safe to reach, you may be able to clear a minor blockage. Do not attempt to access the internal flue system — leave that to an engineer.
- Reset the boiler (up to two or three times) DIY safe
Press the reset button on the boiler and allow it to attempt a full start-up cycle. If the fault was caused by a transient issue — such as a brief wind gust reversing the flue draught — a reset may clear it. Do not reset more than two or three times in quick succession, as repeated failed ignition attempts can indicate a more serious fault.
- Check that your gas supply is on DIY safe
Make sure the gas isolation valve on the supply pipe to the boiler is fully open, and verify that other gas appliances in the property (cooker, hob) are working. If no gas appliances are working, contact your gas supplier rather than resetting the boiler.
- Have an engineer inspect and test the fan Gas Safe engineer
A Gas Safe engineer will check whether the fan starts when the boiler calls for heat, measure its speed, and test whether it is generating the correct pressure. If the fan is faulty, it will need to be replaced. This work must only be carried out by a registered engineer.
- Have an engineer inspect the air pressure switch and its connecting tubes Gas Safe engineer
The engineer will examine the rubber hoses connecting the fan to the monostat for splits, blockages, or kinks, then test the switch itself using a multimeter. If the switch is stuck, damaged, or reading outside its normal range, it will be replaced. Tube integrity checks are quick but must be done by someone competent with the appliance.
- Have an engineer check the wiring and connections Gas Safe engineer
The engineer will inspect the wiring loom between the fan, pressure switch, and PCB — looking for corroded terminals, broken conductors, or loose plugs. Intermittent F.33 faults are particularly often traced back to a poor electrical connection.
- Have an engineer evaluate the PCB if all other components check out Gas Safe engineer
If the fan, pressure switch, tubes, and wiring are all found to be in good order, the PCB may be at fault. The engineer can test the board's outputs with a multimeter. PCB replacement is expensive, so a thorough diagnostic is important before committing to this repair — particularly on older boilers where a replacement may not be cost-effective.
- Contact a Gas Safe registered engineer Gas Safe engineer
If the boiler has not cleared after a visual flue check and one or two resets, book a Gas Safe registered engineer to carry out a full diagnosis. Do not continue attempting resets beyond the recommended number, and do not attempt to access any internal boiler components yourself.
Parts you may need
- Air pressure switch (differential relay / monostat) · from £25
- Boiler fan / flue fan assembly · from £120
- Pressure switch connecting tubes / hoses · from £10
- PCB (printed circuit board) · from £220
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 £80–£320, depending on the underlying cause.
Frequently asked questions
Can I fix the Protherm F.33 fault myself?
There are only a couple of things a homeowner can safely do: check the external flue for an obvious blockage, and attempt a reset (no more than two or three times). Everything beyond that — testing the fan, inspecting the pressure switch and its hoses, checking wiring, or replacing any component — must be done by a Gas Safe registered engineer. Attempting to work inside the boiler yourself is unsafe and will likely invalidate your boiler warranty or home insurance.
How much does it cost to fix a Protherm F.33 fault in the UK?
Most people with this fault end up paying somewhere between £80 and £320 all-in, depending on what is found. Replacing the air pressure switch itself is relatively inexpensive — the part costs around £15–£30 and an engineer can usually fit it within an hour. A fan replacement is the more common and more costly repair, typically running to £150–£300 including labour. If the PCB is at fault, costs can reach £350–£500 or more — at which point, on an older boiler, it is worth getting a quote for a new boiler as a comparison.
Why does the F.33 fault keep coming back after I reset the boiler?
A recurring F.33 that clears temporarily after a reset but returns within a short time usually points to a component that is on the way out rather than fully failed — most often the fan or the pressure switch. An intermittent fault that keeps triggering the lockout needs a proper engineer diagnosis; repeated resetting will not resolve the underlying issue and may mask a worsening problem.
Could a frozen flue cause the F.33 code on my Protherm boiler?
Yes, particularly during cold spells. If ice builds up at the flue terminal, the draught is restricted and the pressure switch may not operate correctly, triggering an F.33 lockout. You can carefully check the external flue outlet for ice and, if it is safe to do so, gently clear it. However, if the blockage is internal or you are unsure, contact an engineer rather than attempting to clear it yourself.