Protherm F.35 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.35 fault code mean?
The F.35 code tells you that your Protherm boiler has detected a fault within its flue or combustion exhaust system. In practical terms, the boiler's internal monitoring has identified a problem that prevents combustion gases from being expelled safely to the outside — whether that's a blockage in the flue, an issue with the fan that pushes gases out, a failed pressure switch, or a wiring fault. Because the F.35 relates to the safe removal of combustion gases (including carbon monoxide), the boiler will lock out and refuse to fire until the underlying problem is resolved. This is a deliberate safety measure. Do not attempt to repeatedly override it.
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 restricted flue outlet Common
The most frequent trigger for F.35 is something physically blocking the flue terminal outside the property — bird nests, accumulated debris, vegetation growing close to the outlet, or even a plastic bag blown against the terminal. When the flue cannot vent freely, the pressure switch detects the abnormal back-pressure and raises the fault.
- Faulty or seized fan (flue fan / inducer) Common
The fan is responsible for drawing combustion gases through the heat exchanger and pushing them out through the flue. If the fan motor has failed, the bearings have seized, the blades are fouled with debris, or the shaft lacks lubrication, air flow will be insufficient and the pressure switch will trip. This is a common cause on older Protherm appliances.
- Failed or sticking air-pressure switch Common
The pressure switch (sometimes called a draft or differential pressure switch) monitors the air flow created by the fan. If the diaphragm inside cracks, the switch sticks in the open position, or the small rubber tube connecting it to the fan housing splits or comes loose, the boiler will see an F.35 even if the fan and flue are perfectly clear.
- Moisture or incorrectly routed pressure-switch tubes Sometimes
Condensate can accumulate inside the small-bore plastic tubes that connect the fan housing to the pressure switch. A waterlogged tube mimics a blockage, preventing the switch from registering correct air pressure. Incorrectly routed tubes — for example, following a repair where the tubes were reconnected the wrong way round — cause the same symptom.
- Broken or corroded wiring to the pressure switch or fan Sometimes
The electrical connections between the pressure switch and the PCB, or between the fan motor and the control board, can develop faults — loose terminals, corroded pins, or a chafed wire. The boiler cannot confirm whether the fan is running correctly and defaults to a flue-fault lockout.
- Flue pipe leak, damage, or excessive length Sometimes
A cracked flue joint, a section that has pulled apart, or a flue run that exceeds the maximum permitted length for the boiler model can all reduce exhaust flow enough to trigger F.35. This is more likely after building work near the boiler or following a DIY flue extension.
- Faulty PCB (control board) Rare
If every other component checks out correctly, the PCB itself may be misreading the pressure-switch signal or generating the fault in error. This is a less common cause but should be considered once the engineer has ruled out all other possibilities.
How to fix it
- Check the external flue outlet for obstructions DIY safe
Go outside and locate where the flue exits the building — usually a circular terminal on an external wall or roof. Look for anything blocking it: bird nests, leaves, debris, or vegetation pressing against the grille. If there is an obvious, safe-to-reach blockage such as a twig or leaves, carefully remove it. Do not attempt to dismantle any part of the flue itself.
- Reset the boiler (once or twice only) DIY safe
With the flue outlet clear, attempt a reset using the boiler's reset button or by following the procedure in your user manual. If the boiler fires and runs normally, monitor it over the next hour. If F.35 returns promptly, do not keep resetting — repeated resets will not fix an underlying mechanical fault and could mask a safety issue.
- Do not attempt to open the boiler casing or work on internal components Gas Safe engineer
The fan, pressure switch, internal flue connections, and all associated wiring are inside the boiler casing. Inspecting or adjusting these components requires Gas Safe registration. Opening the case and interfering with gas-side or combustion-air components without the correct qualifications is both illegal and dangerous.
- Have a Gas Safe engineer inspect the fan and flue system Gas Safe engineer
A registered engineer will remove the casing safely, check fan operation (speed, current draw, blade condition), inspect the internal flue connections for cracks or displaced joints, and verify that the flue run length and configuration comply with the manufacturer's specification. They will clean any debris from the fan inlet if needed.
- Have the engineer test and check the pressure-switch tubes Gas Safe engineer
The engineer should inspect the small-bore tubes running from the fan volute to the pressure switch, drain or clear any accumulated condensate, and confirm the tubes are correctly and securely connected. A cracked or perished tube should be replaced — it is inexpensive but easy to miss.
- Have the engineer test the pressure switch electrically Gas Safe engineer
Using a multimeter, the engineer can confirm the pressure switch opens and closes at the correct differential pressure. A switch that does not actuate within specification should be replaced. The engineer should also check the wiring loom connections at both the switch and the PCB for corrosion or looseness.
- Fan replacement if motor or bearings are found to be defective Gas Safe engineer
If the fan is confirmed faulty — motor windings failed, bearings worn, or blades damaged — the assembly will need to be replaced with a compatible unit. This is a standard repair for a Gas Safe engineer and usually resolves F.35 when the fan is the root cause.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
Because F.35 relates to the safe venting of combustion gases, this fault must be diagnosed and repaired by a Gas Safe registered engineer. You can verify any engineer's registration at GasSafeRegister.co.uk. Do not run the boiler with this fault active — an unresolved flue fault carries a risk of carbon monoxide entering the property.
Parts you may need
- Flue fan / inducer assembly (Protherm compatible) · from £95
- Air pressure switch (differential pressure switch) · from £35
- Pressure switch connecting tubes / hose kit · from £12
- Horizontal flue terminal kit · from £45
- PCB / main control 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 £120–£380, depending on the underlying cause.
Frequently asked questions
Is the Protherm F.35 fault dangerous?
It can be. The F.35 code means the boiler has detected a problem with its flue system — the route by which combustion gases, including carbon monoxide, are expelled outside. The boiler locks out as a precaution, which is actually the safe response. Do not attempt to bypass the lockout or keep resetting repeatedly. Until a Gas Safe engineer has confirmed the flue system is working correctly, do not use the boiler.
Can I fix the F.35 fault myself?
The only homeowner-safe check is inspecting the external flue terminal for obvious blockages such as debris or a bird nest and attempting one or two resets afterwards. Everything else — the fan, pressure switch, internal pipework, and wiring — must be inspected and repaired by a Gas Safe registered engineer. Working on a gas boiler's internal components without the correct qualifications is illegal under UK gas safety regulations and extremely dangerous.
What is the typical repair cost for a Protherm F.35 fault?
Most people with this fault pay somewhere between £120 and £380 in total, covering the call-out, diagnosis, and the most common repairs such as a pressure switch replacement (roughly £80–£160 all-in) or a fan replacement (roughly £200–£350 all-in). If the flue itself needs replacing, costs can rise to £400–£850 depending on the type and complexity of the run. In the rare situation where the PCB is at fault, the board alone can cost £180–£300 plus labour. Always get at least two or three quotes from Gas Safe registered engineers before committing to major parts.
Why does my Protherm keep showing F.35 after I reset it?
If the fault returns quickly after a reset, the underlying problem has not gone away. Common repeat-fault culprits include a fan that is beginning to fail and runs intermittently, a pressure switch with a cracked diaphragm, or a partial flue blockage that restricts flow enough to trip the switch under load. Persistent F.35 faults need a thorough investigation by a Gas Safe engineer rather than repeated resets, which can mask a serious safety issue.