Ferroli F04 Fault Code: Causes, Fixes & Repair Costs
What does the Ferroli F04 fault code mean?
The F04 code on a Ferroli boiler is a safety lockout that points to one of two related problems, depending on your specific model. On older or open-flued Ferroli models, F04 signals that the flue gas thermostat has tripped — meaning the boiler detected excessive flue temperatures and shut itself down for a minimum of 20 minutes to protect the appliance. On newer, sealed-system models, F04 relates to a fault in the fan or air pressure switch circuit: the air pressure switch monitors whether the fan is generating sufficient draught to safely vent combustion gases, and if it cannot confirm this, the boiler refuses to fire. In both cases the boiler is locked out as a safety measure, and a simple reset may not resolve the underlying problem. Check your model documentation if you are unsure which definition applies, but the diagnostic steps and likely causes are broadly the same.
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
- Fan fault or fan failure Common
The fan is responsible for pushing combustion gases safely out through the flue. If the fan motor is running slowly, intermittently, or has failed entirely, the air pressure switch will not detect the correct draught and the boiler will lock out. Fan wear is one of the most common causes of F04 on sealed-system Ferroli models.
- Faulty or stuck air pressure switch Common
The air pressure switch is a small diaphragm-operated component that confirms adequate flue draught before allowing ignition. Over time the diaphragm can split, the switch can stick, or the rubber hose connecting it to the fan housing can crack or become blocked. A faulty switch can trigger F04 even when the fan itself is working correctly.
- Blocked or restricted flue Common
Debris such as leaves, bird nests, snow, or a damaged flue terminal can partially or fully obstruct the flue outlet. This raises back-pressure in the system, preventing the air pressure switch from registering normal operating conditions, or causing flue gas temperatures to rise and trip the flue gas thermostat.
- Flue gas thermostat tripped or faulty Sometimes
On models where F04 specifically relates to the flue gas thermostat, the thermostat may have tripped due to a period of particularly intensive operation, a blocked flue, or inadequate ventilation around the boiler. In some cases the thermostat itself has developed a fault and is triggering the lockout even when temperatures are within safe limits.
- Wiring or connection fault Sometimes
Loose, corroded, or broken wiring between the PCB and the fan, air pressure switch, or flue gas thermostat can interrupt the signal and cause a spurious F04. This is worth a Gas Safe engineer checking before replacing more expensive components.
- Blocked draft breaker or increased pneumatic resistance Sometimes
On open-flued models, a contaminated or obstructed draft breaker can restrict airflow through the combustion circuit. Increased pneumatic resistance anywhere in the flue system — such as mechanical debris within the heat exchanger or flue passages — can replicate the symptoms of a fan or thermostat fault.
- Faulty PCB Rare
If the control board is failing to send the correct signals to the fan or is misreading the air pressure switch input, it can generate an F04 lockout without any mechanical fault being present. PCB failure is less common but should be considered after other causes have been ruled out.
How to fix it
- Reset the boiler once or twice after allowing it to cool DIY safe
If your boiler has been running hard for an extended period, the flue gas thermostat may have tripped due to heat accumulation. Turn the boiler off at the controls, wait at least 20–30 minutes for it to cool, then attempt a reset using the button on the boiler fascia. Do not reset more than two or three times — repeated resets on a locked-out boiler can damage the heat exchanger and fan. If the F04 returns promptly, move on rather than continuing to reset.
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in your home — such as a gas hob or fire — are working normally. If they are not, the issue may be with your gas supply rather than the boiler. Contact your gas supplier if you suspect a supply interruption.
- Visually inspect the flue terminal for obvious blockages DIY safe
Go outside and look at the flue terminal on your external wall or roof. Check for visible obstructions such as a bird nest, accumulated debris, ice, or snow blocking the outlet. Do not insert anything into the flue. If you can see and safely remove an obvious external blockage without accessing the flue pipe itself, do so. If the terminal appears damaged or the blockage is internal, this requires an engineer.
- Check the condensate pipe if your model is a condensing boiler DIY safe
While F04 is not a condensate-specific fault, a frozen or blocked condensate pipe can sometimes contribute to pressure or draught issues. If the pipe running from your boiler to an external drain feels frozen, you can carefully thaw it using warm (not boiling) water poured over the external section. Consult our condensate thawing guide for full instructions.
- Do not attempt to inspect or repair the fan, air pressure switch, flue gas thermostat, wiring, or PCB yourself Gas Safe engineer
All internal components involved in the F04 fault — the fan motor, air pressure switch, flue gas thermostat, wiring loom, and PCB — must only be accessed and worked on by a Gas Safe registered engineer. Working on these parts yourself is unsafe and illegal under UK gas safety legislation. Even checking internal wiring should be left to a qualified professional.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
A Gas Safe engineer will test the fan speed and output, check the air pressure switch and its connecting hose, inspect the flue gas thermostat, examine the wiring between the PCB and all related components, and assess the flue system for restrictions or installation issues. Based on their findings, they will replace whichever component is at fault — commonly the fan, air pressure switch, or flue gas thermostat — and confirm safe operation before leaving. Ask them to check the full flue run, not just the terminal, particularly on older installations.
Parts you may need
- Fan motor assembly (Ferroli compatible) · from £95
- Air pressure switch · from £35
- Flue gas thermostat / overheat thermostat · from £25
- Air pressure switch connecting hose · from £10
- PCB (control board — model specific) · from £180
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 £150–£350, depending on the underlying cause.
Frequently asked questions
Can I just keep resetting the boiler to clear the F04 code?
Resetting once or twice after letting the boiler cool is fine and sometimes resolves a temporary thermostat trip. However, if the F04 returns each time you reset, there is an underlying mechanical fault that resetting will not fix. Repeatedly forcing the boiler to attempt ignition when the fan or air pressure circuit is faulty can overheat the heat exchanger or burn out the fan motor, turning a relatively affordable repair into a much more expensive one. Two or three resets is the sensible maximum before calling an engineer.
How much does it cost to fix a Ferroli F04 fault in the UK?
For the most common causes — fan replacement or air pressure switch replacement — expect to pay roughly £150–£350 including parts and labour. A flue gas thermostat replacement is typically at the lower end, around £100–£150. Fan replacement usually falls in the £180–£250 range. If the PCB turns out to be at fault, costs rise: the board itself can cost £150–£300 depending on the model, plus £100–£150 labour, so a PCB repair can exceed £400 on some Ferroli models. If your boiler is over 10 years old and needs a PCB or fan, it is worth getting a replacement boiler quote at the same time to compare long-term value.
Why does my Ferroli show F04 but the fan sounds like it is running?
The fan may be spinning but not running at the correct speed or generating sufficient pressure to satisfy the air pressure switch. A worn fan motor can rotate but lack the power to create adequate draught, especially when cold. Alternatively, the air pressure switch itself, or the small rubber hose connecting it to the fan housing, may be faulty — meaning the switch is not registering the correct pressure even though the fan is working properly. A Gas Safe engineer can test both components individually to identify which is at fault.
Is the Ferroli F04 dangerous — should I be worried about carbon monoxide?
The F04 lockout is the boiler's safety system working as intended: it has detected a problem with flue draught or flue gas temperatures and shut down rather than continue operating unsafely. While the lockout itself means combustion gases are not being produced, the underlying fault — a fan failure or blocked flue — is precisely the kind of issue that can lead to dangerous flue gas spillage if the boiler were to operate without proper venting. Fit working carbon monoxide alarms on every floor of your home if you have not already done so, and do not attempt to override the lockout. Get the boiler assessed by a Gas Safe engineer promptly.