Ferroli A51 Fault Code: Causes, Fixes & Repair Costs
What does the Ferroli A51 fault code mean?
The A51 code indicates that your Ferroli boiler has detected a poor combustion condition — in plain terms, either the burner failed to light properly or the boiler lit but the flame sensor could not confirm a stable flame was present. As a result, the boiler has shut itself down as a safety precaution. This is a hard lockout fault (Ferroli 'A' codes always are), meaning the boiler will not restart on its own — a manual reset is required at minimum, though the underlying cause must be identified and resolved to prevent the fault returning. Do not confuse this with Ferroli 'F' prefix codes, which are softer blocking faults that can clear automatically once the triggering condition improves.
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
- Faulty or contaminated flame sensor (ionisation electrode) Common
The flame sensor detects whether a stable flame is present by measuring a small electrical current through the flame itself. If the electrode tip is coated in combustion deposits or residue, or if the electrode is cracked, incorrectly positioned, or has a loose wiring connection, it can fail to send a reliable signal — causing the boiler to conclude no flame exists even when it has lit. This is the single most common cause of A51 on these models.
- Ignition electrode fault or misalignment Common
Separate from the flame sensor, the ignition electrode generates the spark that lights the gas. If it is worn, cracked, or has drifted out of position relative to the burner, the boiler may simply fail to ignite at all, resulting in a flame failure lockout. Damaged high-tension leads or loose connections at the electrode can produce the same result.
- Restricted air inlet or flue outlet Sometimes
Combustion requires a steady supply of fresh air and an unobstructed route for exhaust gases to exit. A partially blocked flue terminal — caused by debris, bird nesting material, or ice in winter — or a kinked/crushed air inlet pipe can starve the burner of oxygen, producing unstable or incomplete combustion that the sensor cannot confidently read as a healthy flame.
- Gas valve fault or reduced gas pressure Sometimes
If the gas valve is not opening fully, is sticking, or the incoming gas pressure from the supply is lower than the boiler requires, the burner may produce a weak or unstable flame that fails the ionisation check. This can also occur transiently if a neighbouring appliance draws heavily on the supply simultaneously, though a persistent fault points to a component issue rather than supply fluctuation.
- Trapped air in the gas line Rare
After a period of no use, maintenance work, or a gas supply interruption, air can become trapped in the gas pipework feeding the boiler. The first ignition attempt may fail as air purges through, and if the boiler locks out before gas reaches the burner the A51 code can appear. A single careful reset will often resolve this specific scenario.
How to fix it
- Attempt a single boiler reset DIY safe
Locate the reset button on the front of the boiler — on most Ferroli models it is clearly labelled. Press and hold it for approximately 10 seconds until the boiler attempts to restart. Watch whether the burner lights and the fault code clears. If it does, monitor the boiler closely over the next hour to confirm the fault does not return. If the A51 reappears, do not keep resetting — repeated forced ignition attempts can damage the heat exchanger and fan, turning a minor repair into a much costlier one. Two to three attempts in total is a sensible limit.
- Check that your gas supply is active DIY safe
Before calling an engineer, quickly confirm that gas is reaching your home by trying another gas appliance — a cooker hob burner is ideal. If it lights normally, the supply is working and the issue lies within the boiler itself. If no gas appliances work, check whether a gas meter emergency control valve has been accidentally closed, and contact your gas supplier if the supply appears to be off at the meter.
- Inspect the flue terminal and air inlet for obvious blockages DIY safe
From outside your property, take a look at the flue terminal (the white plastic fitting where the exhaust exits the wall). Check for any visible obstruction such as leaves, nesting material, ice, or physical damage to the terminal grille. Do not attempt to dismantle the flue or reach inside it — this is simply a visual check. If you can see a clear blockage at the terminal, report this to the engineer when you call.
- Arrange a visit from a Gas Safe registered engineer for internal inspection and repair Gas Safe engineer
All work inside the boiler casing involving combustion components requires a Gas Safe registered engineer. The engineer will remove the casing and inspect the ionisation electrode for carbon deposits, cracking, or misalignment, cleaning or replacing it as needed. They will also check the ignition electrode and high-tension leads, verify gas valve operation and inlet pressure, and check the flue and air pathways for restriction. After completing any repairs they will carry out combustion analysis to confirm the boiler is burning safely before returning it to service. You can verify any engineer's Gas Safe registration at the official Gas Safe Register website before booking.
Parts you may need
- Ionisation / flame sensing electrode · from £25
- Ignition electrode · from £20
- Ignition lead (high-tension cable) · from £15
- Gas valve · from £120
- Flue terminal assembly · from £35
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–£300, depending on the underlying cause.
Frequently asked questions
Can I just keep resetting the boiler to clear the A51 code?
A single reset is a perfectly reasonable first step and will fix the problem if the cause was something transient like a brief gas supply interruption or trapped air. However, if the A51 returns after resetting, that tells you there is a genuine fault that needs a professional diagnosis. Repeatedly resetting a boiler with an active combustion fault puts extra thermal stress on the heat exchanger and fan, and can turn a relatively affordable repair (such as a new flame sensor) into a much more expensive one. Two or three reset attempts is a sensible limit before calling an engineer.
What is the difference between Ferroli 'A' and 'F' fault codes?
Ferroli uses two distinct fault code prefixes to indicate how serious a fault is and how the boiler responds to it. Codes beginning with 'F' are blocking faults — the boiler shuts down but will attempt to restart automatically once the condition that triggered the fault resolves itself. Codes beginning with 'A', such as A51, are lockout faults — the boiler shuts down completely and will not restart under any circumstances until someone physically presses the reset button. Because A51 is a lockout, it always requires manual intervention and will never self-resolve.
How much should I expect to pay to fix an A51 fault in the UK?
For the most common repair — cleaning or replacing the flame sensing electrode — you are typically looking at £100–£180 including the engineer's call-out, labour, and parts. If the ignition electrode or leads also need attention the cost is similar. A gas valve replacement is more involved and typically costs £250–£350 all-in. In rare cases where the PCB or control board is implicated, costs can reach £300–£450, though this is not the usual cause of A51. Ferroli parts tend to be reasonably priced, but availability can vary on older models and may extend lead times slightly.
My Ferroli boiler is over 10 years old and keeps showing A51 — should I repair or replace it?
If the A51 fault is appearing regularly alongside other recurring faults, and your boiler is more than 10–12 years old, it is worth asking your engineer to quote for both repair and replacement at the same visit. As a rough rule of thumb, if repair costs in a given year are likely to exceed around a third of what a new A-rated boiler would cost installed, replacement usually makes better long-term financial sense — especially given the energy efficiency gains a modern condensing boiler offers. Your engineer can give you an honest assessment of the boiler's overall condition.