Ideal FN Fault Code: Causes, Fixes & Repair Costs
What does the Ideal FN fault code mean?
The FN fault code on an Ideal boiler indicates that the flame has been lost during operation. The boiler's PCB (printed circuit board) monitors a flame sensor (ionisation probe) throughout every heating cycle. If that sensor reports that the burner flame has gone out — or failed to establish and hold — the PCB immediately puts the boiler into safety lock-out and displays FN on the front panel. The FN code is closely related to the F2 and LN codes, all three pointing to flame-loss events; the root cause is typically a problem with gas supply, the gas valve, the ignition or flame-sensing electrode, the flue and fan system, or a blocked condensate pipe.
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
- Gas supply interrupted or insufficient Common
If the boiler is not receiving adequate gas pressure, the burner cannot sustain a flame. Common culprits include a pay-as-you-go meter that has run out of credit, an isolation valve under the boiler or at the meter that has been partially or fully closed (perhaps after recent maintenance), or low incoming gas pressure from the street supply.
- Faulty or sticking gas valve Common
The gas valve controls how much gas is delivered to the burner. If it is stuck, worn, or not opening to the correct position, gas flow is restricted below the level needed to keep the flame alight. In some cases an engineer can free a stuck valve without replacing it, but a fully failed valve will need swapping out.
- Worn or dirty flame sensor or ignition electrode Common
The ignition electrode produces the spark to light the gas, while the flame sensor (ionisation probe) confirms the flame is burning. Carbon deposits or electrode erosion — both common after years of use — can cause unreliable sparking or a weak flame signal. The boiler may light briefly before locking out with FN because the PCB cannot confirm a stable flame.
- Fan or flue system fault Sometimes
The fan clears combustion gases and maintains the correct flue draught for safe burning. A sluggish or failing fan, a blocked flue terminal (debris, bird nest, heavy icing), or wind blowing back down the flue can all disrupt combustion enough to extinguish the flame and trigger a lock-out. Unusual grinding or rattling noises when the boiler tries to fire can be a sign the fan is struggling.
- Blocked or frozen condensate pipe Sometimes
Condensing boilers produce acidic water vapour that drains away through a condensate pipe, usually running to an outside drain. In cold weather this pipe can freeze solid; at any time of year a partial blockage from debris can restrict flow. The resulting back-pressure affects the air pressure switch, which the PCB interprets as an unsafe condition and triggers a lock-out — sometimes displaying FN.
How to fix it
- Check your gas supply is live DIY safe
Test another gas appliance in the property (a gas hob, for example). If nothing else is working either, contact your gas supplier — there may be a street-level supply issue or a meter credit problem. If you have a pay-as-you-go meter, top it up and confirm credit is showing before proceeding.
- Confirm the gas isolation valve is fully open DIY safe
Locate the yellow-handled isolation valve on the gas pipe beneath the boiler. When the handle is pointing in line with the pipe, gas is flowing. If it is at a 90-degree angle to the pipe it is closed — open it fully and try again. Do not attempt to adjust any other valves on the gas pipework.
- Check the flue terminal outside DIY safe
Go outside and visually inspect the flue outlet. Look for obvious blockages such as a bird nest, a build-up of ice, or accumulated debris around the terminal. Do not attempt to reach or dismantle any part of the flue; if you spot a blockage you cannot safely clear from ground level, note it and report it to the engineer.
- Check for a frozen condensate pipe DIY safe
In freezing weather, find the condensate pipe — typically a white or grey plastic pipe (often 21.5 mm or 32 mm diameter) that exits the boiler and runs to an external drain or gulley. If it feels solid or is visibly iced over, you can carefully thaw it by pouring warm (not boiling) water over the external section, or wrapping it with a hot-water bottle. Once thawed, reset the boiler once and observe. If the pipe freezes repeatedly, an engineer should reposition or insulate it properly.
- Reset the boiler — once only DIY safe
Press and hold the reset button (refer to your boiler manual for the exact location — on most Ideal models it is clearly labelled on the front panel) for a few seconds until the display changes. Allow the boiler to go through its full start-up sequence. If it locks out again with FN, do not keep resetting it; repeated resets without resolving the underlying fault can be unsafe and can mask a potentially dangerous condition.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
If the boiler continues to display FN after your checks, a Gas Safe registered engineer must carry out the repair. They will test gas pressure at the valve, inspect and clean or replace the flame sensor and ignition electrode, check the gas valve for correct operation, assess the fan and flue system, and verify there are no condensate blockages. Do not attempt to work on the gas valve, electrodes, PCB, or any internal gas components yourself.
Parts you may need
- Ignition / flame sensing electrode · from £35
- Gas valve · from £120
- Boiler fan · from £90
- HT (high-tension) ignition lead · from £20
- Condensate trap · from £18
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–£350, depending on the underlying cause.
Frequently asked questions
Is the Ideal FN fault code the same as F2 or LN?
They are closely related. All three — FN, F2, and LN — indicate a flame-loss event on Ideal boilers, and the root causes overlap significantly. The subtle differences relate to which part of the firing sequence the flame was lost, but in practice you will follow the same diagnostic process for all three. If you see any of these codes, the guidance on this page applies.
Can I fix the Ideal FN fault code myself?
A handful of checks are safe for a homeowner: verifying the gas supply is on, thawing a frozen condensate pipe, checking the flue terminal for obvious blockages, and performing a single reset. Beyond that, the fault almost certainly involves a gas component — the valve, electrodes, or combustion system — and UK law requires a Gas Safe registered engineer to work on those parts. Attempting DIY repairs on gas components is illegal and potentially very dangerous.
How much does it cost to fix an Ideal boiler FN fault code?
Most people pay somewhere between £100 and £350 all in, depending on what needs doing. A straightforward electrode clean or replacement with a call-out tends to sit toward the lower end; a gas valve replacement will typically cost more, often £250–£350 including parts and labour. Fan replacement can also run to the higher end of this range. In rare cases where the PCB has failed or the heat exchanger is damaged, costs can climb significantly higher — if repair quotes start approaching the cost of a new boiler and your boiler is over 12–15 years old, it is worth getting a new-boiler quote for comparison.
Why does my Ideal boiler keep showing FN after I reset it?
A reset clears the lock-out signal but does not fix the underlying problem. If FN returns quickly, it means the root cause — most often a gas pressure issue, a faulty valve, or a worn electrode — is still present. Repeatedly resetting is not a safe workaround and can mask a dangerous condition. If the boiler locks out again after a single reset, stop resetting it and arrange for a Gas Safe engineer to investigate.