Ferroli A04 Fault Code: Causes, Fixes & Repair Costs
What does the Ferroli A04 fault code mean?
The A04 is a lockout fault on Ferroli boilers, meaning the boiler has shut itself down and will not restart until you manually reset it. In plain terms, your boiler has detected a serious problem with the way it is handling flue gases — either dangerously high flue gas temperatures, a fault with the flue gas removal system, or both occurring repeatedly. On most Ferroli models (including the Modena, Bluehelix, and i-Series ranges), A04 specifically means that the underlying F07 fault — high fume temperature detected by the flue probe — has been triggered three or more times within a 24-hour period. On Domiproject D and C models, it points directly to overheating of the flue gas thermostat. On the Optimax HE Plus 38C it indicates activation of the fume extraction duct safety device. In all cases the boiler is telling you it cannot safely remove combustion gases, and it will stay locked out until the root cause 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 restricted flue terminal Common
A partial or full blockage at the external flue terminal — caused by bird nests, debris, or ice in cold weather — prevents combustion gases from escaping freely. Back-pressure builds up, flue temperatures rise, and the safety circuit triggers F07 repeatedly until A04 locks the boiler out.
- Faulty or failing flue gas thermostat Common
The flue gas thermostat monitors the temperature of gases leaving the heat exchanger. If its contacts open at the wrong time — due to age, heat stress, or a wiring fault — the boiler interprets this as a dangerous overheating event and eventually locks out with A04.
- Fan fault or deteriorating fan performance Common
The fan is responsible for drawing combustion gases through the heat exchanger and out of the flue. If the fan is running slowly, intermittently, or not at all, gases linger longer than they should, raising their temperature. A Gas Safe engineer will need to test fan speed and current draw.
- Air pressure switch fault Sometimes
The air pressure switch confirms that the fan is generating sufficient draught before allowing ignition. A switch that is sticking, has a blocked sensing tube, or has drifted out of calibration can falsely signal that flue gas removal is inadequate, triggering the F07 condition that leads to A04.
- Incorrectly sized or poorly installed flue Sometimes
If the flue run is too long, has too many bends, or was installed with the wrong diameter pipe, the system will never perform correctly. This is usually a pre-existing installation issue that worsens as the boiler ages or as flue components corrode internally.
- PCB (printed circuit board) fault Rare
In rarer cases, the control board itself misreads signals from the flue thermostat or fan circuit, generating spurious F07 events until A04 is triggered. PCB faults are harder to diagnose and are usually only confirmed after other components have been ruled out.
How to fix it
- Reset the boiler once DIY safe
Locate the reset button on your Ferroli — it is typically marked with an 'R' symbol or a flame icon on the control panel. Press and hold it for around 5–10 seconds, then release. Allow the boiler a full minute to attempt a restart. If it fires up and runs normally, monitor it closely over the next 24 hours. A single successful reset after a genuine one-off event is acceptable, but if the fault returns, move on to the checks below rather than repeatedly resetting.
- Check the external flue terminal for visible blockages DIY safe
From a safe position at ground level, look at the flue outlet on the outside wall or roof. Check for obvious obstructions such as bird nests, leaves, ice build-up, or physical damage to the terminal guard. Do not climb ladders or access the roof yourself. If you can see a clear blockage and safely reach it, you may carefully clear it, but if the terminal is at height or the blockage is inside the flue, this is a job for an engineer.
- Check whether the condensate pipe is frozen DIY safe
In cold weather, a frozen condensate pipe can indirectly cause flue-related faults. Locate the white plastic condensate pipe that runs from the boiler, usually to an external drain or gulley. If it feels solid or you can see ice, thaw it gently using warm (not boiling) water poured along the pipe, or wrap it with a warm cloth. Once thawed, reset the boiler. Consider insulating the pipe to prevent a recurrence.
- Do not reset the boiler more than two or three times in total DIY safe
If the A04 fault returns after one or two resets, stop resetting. Repeated lockouts signal an underlying fault that requires professional diagnosis. Continuing to reset without addressing the cause risks further damage and is potentially unsafe.
- Arrange inspection of the flue gas thermostat and associated wiring Gas Safe engineer
A Gas Safe registered engineer will inspect the flue gas thermostat — checking whether its contacts are opening at the correct temperature, whether the cable and connections are secure, and whether the component itself needs replacing. This is not a homeowner task as it involves working on internal boiler components.
- Have the fan and air pressure switch tested professionally Gas Safe engineer
The engineer will measure fan motor current draw and speed to confirm it is operating within specification. The air pressure switch sensing tube will be checked for blockages or kinks, and the switch itself tested. Either or both components may need replacing.
- Commission a full flue integrity check Gas Safe engineer
The engineer should carry out a flue flow and integrity check to confirm that combustion products are being safely removed. They will inspect the internal flue run for corrosion, incorrect joints, or sizing issues. This check also confirms whether the flue terminal positioning complies with current Building Regulations.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
If the boiler has not responded to a reset, or the A04 fault keeps returning, book a Gas Safe registered engineer as soon as possible. Ferroli boilers that lock out repeatedly on a flue gas fault should not be used until the cause has been identified and rectified. You can verify an engineer's registration at gassaferegister.co.uk.
Parts you may need
- Flue gas thermostat · from £35
- Boiler fan / flue fan assembly · from £95
- Air pressure switch · from £45
- Flue gas temperature sensor / NTC probe · from £25
- PCB (control board) · from £150
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–£280, depending on the underlying cause.
Frequently asked questions
Can I keep resetting my Ferroli boiler to clear the A04 fault?
You can try one or two resets to see whether the fault was a temporary one-off — for example caused by a brief flue restriction on a particularly windy day. However, A04 is specifically triggered when the underlying F07 condition has already occurred three times in 24 hours, so by the time you see A04 your boiler has already been struggling for a while. Repeatedly resetting without fixing the root cause is not a safe long-term approach and can mask a worsening problem. If the fault returns after a reset, call a Gas Safe engineer.
What is the F07 fault on a Ferroli boiler, and how does it relate to A04?
F07 is a blocking fault that appears when the flue gas temperature probe detects an excessively high temperature during or after a heating cycle. Unlike A04, an F07 may clear by itself once the boiler cools down. However, once F07 has fired three times within a 24-hour window, Ferroli's control system escalates this to an A04 lockout, which requires a manual reset and — more importantly — professional attention to find out why the flue gas temperature keeps spiking.
How much does it typically cost to fix a Ferroli A04 fault in the UK?
For the most common repairs, most homeowners pay somewhere between £100 and £280 all-in. A sensor or thermostat replacement sits at the lower end (roughly £100–£150 including labour), while fan replacement tends to cost around £180–£250. If the PCB turns out to be at fault, budget closer to £200–£300. Bear in mind that the initial call-out and diagnosis alone can be £80–£120, and that is often credited against the repair cost if you proceed. In rare cases where the flue itself needs rerouting or replacing, costs can exceed £400, but this is unusual.
My Ferroli boiler is over 10 years old and keeps throwing the A04 code — should I repair or replace it?
This is a common dilemma. As a rough guide, if the repair quote is approaching 30–40% of the cost of a new boiler, replacement usually makes more financial sense — especially if the boiler has had other faults recently or is no longer under any warranty. A new A-rated condensing boiler will also be considerably more efficient than an older Ferroli, reducing your energy bills. Get a repair quote from a Gas Safe engineer first, but do ask them for an honest opinion on the overall condition of the unit at the same time.