Ferroli A21 Fault Code: Causes, Fixes & Repair Costs
What does the Ferroli A21 fault code mean?
The A21 is a lockout fault on Ferroli boilers, meaning the appliance has shut itself down completely and will not restart until the underlying problem is addressed and the boiler is manually reset. It indicates that the boiler's control system has detected unacceptably poor combustion quality — in other words, the burner is not burning gas cleanly or efficiently enough to meet the boiler's safety thresholds. On models such as the DIVAtech and DOMIcondens F series, this lockout is triggered automatically after the related blocking fault (F20) has occurred six times within a ten-minute window, confirming the issue is persistent rather than a one-off blip. The root causes range from flue or condensate blockages through to a faulty ionisation electrode, gas valve, fan, or PCB.
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
- Flue obstruction or incorrect flue sizing Common
A partially or fully blocked flue terminal prevents combustion gases from escaping freely, upsetting the air-to-gas ratio inside the heat exchanger and triggering poor combustion detection. Bird nests, debris, and ice build-up at the external flue terminal are common culprits, as is a flue that was never sized correctly for the appliance.
- Faulty or contaminated ionisation electrode Common
The ionisation electrode is a slender probe that monitors whether a stable flame is present during burner operation. If the electrode is coated in sooty deposits, positioned incorrectly, or its wiring has deteriorated, it sends an unreliable signal to the PCB. The boiler interprets this as combustion failure and locks out even when the burner may actually be firing.
- Frozen or blocked condensate pipe Common
During cold weather the plastic condensate pipe — particularly any section routed externally — can freeze solid. This prevents acidic condensate from draining away, causing it to back up into the sump and ultimately disrupting the combustion airflow through the boiler. This is one of the more straightforward causes to identify and resolve without an engineer.
- Fan not running or running slowly Sometimes
The combustion fan is responsible for drawing in fresh air and expelling waste gases. If the fan fails to start on reset, runs below its target speed, or is obstructed by debris, there is insufficient airflow across the burner. The boiler detects the resulting combustion imbalance and locks out on A21.
- Gas valve fault or insufficient gas supply Sometimes
A gas valve that is not opening fully, or is delivering gas at the wrong pressure, means the burner receives too little fuel relative to the available air. Poor combustion quality follows. Less commonly, an issue with the property's gas supply — such as a meter fault or supply interruption — can produce the same effect.
- Wiring or PCB fault Rare
Damaged cable insulation, a corroded connector, or a failing printed circuit board can send false or erratic signals across the combustion monitoring circuit. In these cases the boiler locks out on A21 not because combustion is genuinely poor, but because the electronics cannot read it accurately. PCB failure is typically a last-resort diagnosis reached only after other causes have been ruled out.
How to fix it
- Check your gas supply is active DIY safe
Confirm that other gas appliances in your home — a hob or gas fire, for example — are working normally. If nothing gas-powered is working, contact your gas supplier rather than continuing to reset the boiler. Also check that your gas meter's emergency control valve is fully open (handle parallel to the pipe).
- Inspect the external flue terminal for obstructions DIY safe
Go outside and look at where the flue exits the building. The terminal should be clear of debris, ice, bird nests, or any object that might restrict airflow. Do not attempt to dismantle the flue; if the terminal itself appears damaged or if the obstruction cannot be removed safely from the outside, leave it for an engineer.
- Check and thaw a frozen condensate pipe (winter months) DIY safe
In cold weather, locate the white plastic condensate pipe — usually 21.5 mm or 32 mm in diameter — that runs from the boiler to an external drain or soakaway. If it feels solid or has visible ice, gently apply warm (not boiling) water along the pipe using a watering can or hot-water bottle until it flows freely again. Once thawed, reset the boiler and monitor it.
- Reset the boiler — but no more than two or three times DIY safe
Press and hold the reset button on the boiler fascia as described in your user manual, typically for three to five seconds. Allow the boiler a full start-up cycle before judging whether the fault has cleared. If A21 returns after one or two resets, stop resetting — repeated forced restarts when combustion is genuinely compromised can damage the heat exchanger and fan, turning a moderate repair into a costly one.
- Have a Gas Safe engineer inspect the ionisation electrode Gas Safe engineer
A registered engineer will remove the burner assembly to access the ionisation electrode, check its gap and orientation against Ferroli's specification, clean off any carbon deposits, and test the electrode circuit for continuity. If the electrode is cracked or the wiring is burnt, it will be replaced. This is not a task for a homeowner — it requires burner removal and proximity to gas-carrying components.
- Have a Gas Safe engineer check and test the combustion fan Gas Safe engineer
The engineer will verify that the fan starts promptly on a call for heat, reaches its correct operating speed, and that the air-pressure differential switch is confirming this to the PCB. A sluggish or non-starting fan will be replaced. Ferroli fan units are generally available as service parts at a reasonable cost.
- Have a Gas Safe engineer inspect the gas valve and carry out a combustion analysis Gas Safe engineer
Using a calibrated flue-gas analyser, the engineer will verify that the boiler is burning gas within Ferroli's specified CO₂ and CO parameters. If combustion figures are out of range despite clean electrodes and clear flue paths, the gas valve will be tested for correct inlet and outlet pressures. A valve that cannot be set correctly must be replaced.
- If all components test satisfactorily, have the PCB assessed Gas Safe engineer
Where no mechanical or combustion cause can be found, the engineer will carry out PCB diagnostics. Control boards on Ferroli boilers are replaceable as a unit. Because PCBs are relatively expensive and sometimes need to be sourced specially for older models, a reputable engineer will confirm the diagnosis thoroughly before ordering one.
- Contact a Gas Safe registered engineer if the fault persists or returns Gas Safe engineer
If the A21 code keeps coming back after resetting, or if you were unable to identify and clear an obvious cause such as a frozen condensate pipe, book a Gas Safe registered engineer. You can verify any engineer's registration at the Gas Safe Register website (gassaferegister.co.uk). Do not continue operating the boiler in lockout.
Parts you may need
- Ionisation / detection electrode · from £25
- Combustion fan assembly · from £95
- Gas valve · from £120
- Printed circuit board (PCB) · from £150
- Condensate trap assembly · from £20
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–£350, depending on the underlying cause.
Frequently asked questions
Can I fix the Ferroli A21 fault code myself?
The only homeowner-safe checks are confirming your gas supply is on, inspecting the external flue terminal for visible blockages, thawing a frozen condensate pipe, and resetting the boiler once or twice. Everything beyond that — electrode inspection, fan testing, gas valve work, or PCB diagnostics — must be carried out by a Gas Safe registered engineer. Working on combustion components without registration is illegal in the UK and potentially dangerous.
Why does my Ferroli keep showing A21 shortly after I reset it?
If A21 returns within minutes of a reset, the underlying fault is still active. The most common repeated causes are a partially blocked flue, a failing ionisation electrode giving an unstable signal, or a fan that starts briefly but does not sustain the correct airflow. Stop resetting and book an engineer — repeated lockout-and-reset cycles put unnecessary strain on the heat exchanger and fan motor.
What is the difference between the Ferroli A21 and F20 codes?
F20 is a blocking fault for poor combustion on certain Ferroli models such as the DIVAtech and DOMIcondens F series — the boiler pauses but may attempt to restart automatically once conditions improve. A21 is the escalated lockout version: if F20 is triggered six times within ten minutes, the boiler decides the problem is persistent and locks out completely, requiring a manual reset before it will try again. If you are seeing A21, the issue has already repeated itself multiple times in a short period.
How much does it typically cost to repair a Ferroli A21 fault in the UK?
Most A21 repairs in the UK fall between £120 and £350 all-in, covering the engineer's call-out, labour, and the most commonly replaced parts such as the ionisation electrode (around £25 for the part) or the fan assembly (around £95 for the part). A gas valve replacement sits toward the upper end of that range. PCB replacement is less common but can push costs higher — typically £300 to £450 including labour — and is worth weighing against the age of the boiler. Ferroli parts are generally more affordable than those for premium brands, though availability on discontinued models can extend lead times.