Protherm F.05 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.05 fault code mean?
The F.05 (or F5) fault code on a Protherm boiler has two distinct meanings depending on which model you own. On most Protherm boilers — including the Cheetah, Bear, Gepard, Panther, and Lynx Combi — F.05 signals an air flow or flue pressure fault, meaning the boiler's control system has detected insufficient air movement to operate safely. This is typically linked to the fan, air pressure switch, flue system, or associated pipework. On models equipped with an outdoor weather-compensation (equithermal) sensor — such as certain Panther and Leopard variants — F.05 instead indicates a fault with that external temperature sensor. The code will only appear for the sensor fault if an outdoor sensor is actually wired into the system. Check your model's documentation to confirm which interpretation applies to your boiler.
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 partially obstructed flue terminal Common
Debris such as leaves, bird nesting material, or ice can partially or fully block the flue outlet, restricting exhaust airflow. In winter, a frozen coaxial flue terminal is a particularly common trigger. The boiler detects reduced pressure differential and locks out as a safety measure.
- Faulty or sticking air pressure switch Common
The air pressure switch monitors airflow by detecting the pressure difference created by the fan. If the switch diaphragm becomes damaged, its hose connections split, or the contacts corrode, it can fail to signal that adequate airflow is present — even when the fan itself is working correctly.
- Fan failure or degraded fan performance Common
The combustion fan pushes flue gases out of the boiler and draws in fresh air for combustion. Motor bearing wear or burnt-out windings can cause the fan to run slowly or stop altogether, triggering the F.05 airflow fault.
- Faulty or disconnected outdoor NTC sensor (equithermal models only) Sometimes
On Protherm models with weather-compensation control, a short circuit, open circuit, or corroded connector at the outdoor sensor will cause the boiler to log F.05. The boiler may automatically revert to using its internal water temperature sensor while the fault is active.
- Flue duct sizing or installation issue Sometimes
If the flue run is too long, the diameter is undersized for the application, or there are too many bends, the resulting restriction in airflow can cause persistent F.05 faults — particularly at high firing rates.
- Mains voltage instability Rare
Significant voltage surges or dips on the electrical supply can cause sensor readings to appear erratic, leading the boiler's electronics to interpret the anomaly as a component failure and lock out with F.05.
- PCB or control module fault Rare
In rare cases, the boiler's main PCB may misread or fail to process signals from the fan, pressure switch, or outdoor sensor correctly, generating a spurious F.05 even when all associated components are functioning normally.
How to fix it
- Check for an obvious flue blockage or ice at the terminal DIY safe
Go outside and visually inspect the flue outlet. Look for bird debris, leaves, ice, or any visible obstruction. In cold weather, a frozen coaxial terminal is a common culprit — carefully thawing it with warm (not boiling) water can resolve the fault immediately. Do not use a naked flame.
- Ensure adequate ventilation in the boiler room DIY safe
If your boiler has an open flue or draws combustion air from the room, open a window or door near the boiler briefly. A severely under-ventilated space can restrict airflow enough to trigger the fault on some models.
- Reset the boiler and observe DIY safe
After checking the flue, reset the boiler using the reset button as described in your owner's manual. Allow the boiler to attempt a full start cycle and note whether the fault returns immediately or after a period of running. Limit resets to two or three attempts — repeated unsuccessful resets will not fix an underlying fault and may mask the problem.
- Check outdoor sensor wiring and connections (equithermal models only) DIY safe
If your Protherm is fitted with an outdoor weather-compensation sensor, locate the sensor and its wiring run. Look for visible damage, loose plug connections, or signs of moisture ingress at the connector. A loose plug can often be reseated by hand. Do not probe live electrical terminals.
- Have a Gas Safe engineer inspect and test the air pressure switch and hoses Gas Safe engineer
A registered engineer will use a manometer or multimeter to check whether the pressure switch is opening and closing at the correct differential, and will inspect the silicone hoses connecting it to the fan and venturi for splits, blockages, or incorrect routing. A faulty switch is straightforward to replace with the correct Protherm part.
- Have the fan tested and replaced if necessary Gas Safe engineer
The engineer will check fan speed, motor current draw, and electrical connections. A fan running below specification or intermittently will need replacing. Fan replacement on Protherm boilers typically requires draining relevant sections and full recommissioning.
- Have the outdoor NTC sensor resistance tested and replaced if required Gas Safe engineer
Using a multimeter, the engineer will check the sensor's resistance against the expected values at the ambient temperature. A reading outside the accepted range confirms sensor failure. Replacement sensors are generally inexpensive, with most of the cost being labour.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If none of the above checks resolves the F.05, the fault may lie with the PCB, flue system design, or a combination of issues that require specialist diagnostic equipment. Always verify the engineer holds current Gas Safe registration at gassaferegister.co.uk before allowing work to proceed.
Parts you may need
- Protherm air pressure switch · from £55
- Protherm combustion fan assembly · from £130
- Outdoor NTC temperature sensor · from £35
- Air pressure switch hose / tubing · from £12
- Protherm PCB / 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 £100–£375, depending on the underlying cause.
Frequently asked questions
My Protherm shows F.05 — do I need a Gas Safe engineer or can I fix it myself?
For most causes of F.05 — a faulty fan, air pressure switch, or blocked flue — you will need a Gas Safe registered engineer to diagnose and repair the fault safely. The one thing you can safely do yourself is check the flue terminal for obvious blockages or ice and attempt a reset or two. If the code returns, stop resetting and book an engineer. Never attempt to access internal gas or electrical components yourself.
Why does the F.05 fault mean different things on different Protherm models?
Protherm uses a shared fault-code numbering system across a wide range of boilers, but the underlying hardware varies significantly between models. On sealed-flue and open-flue boilers with fan-assisted combustion (Cheetah, Bear, Gepard, Lynx Combi), F.05 monitors the air pressure circuit. On models with optional weather-compensation control (certain Panther and Leopard variants), the same code number is used to flag an outdoor sensor fault. Always cross-reference with your specific model's installation or user manual to confirm which meaning applies.
How much does it cost to fix a Protherm F.05 fault in the UK?
Most F.05 repairs fall between £100 and £375. A simple air pressure switch replacement typically costs £140–£200 including parts and labour. Fan replacement usually runs £225–£375. An outdoor sensor swap is generally less expensive at around £80–£150. Bear in mind that call-out or diagnostic fees of £50–£100 are common and may or may not be credited against the repair cost — confirm this with your engineer beforehand. In rare cases where the PCB is at fault, costs can rise to £400 or more, but this is not the typical outcome for F.05.
Can a frozen flue cause the Protherm F.05 code in winter?
Yes — this is one of the most common reasons for F.05 appearing during cold spells. On coaxial (concentric) flue systems, condensate and moisture in the exhaust can freeze at the terminal, partially or fully blocking airflow. You can carefully apply warm water to the terminal to thaw it. Once clear, reset the boiler. If it happens repeatedly, an engineer can fit a frost-resistant terminal or reposition the flue exit to a less exposed location.