Ferroli F34 Fault Code: Causes, Fixes & Repair Costs
What does the Ferroli F34 fault code mean?
The F34 code on a Ferroli boiler has two distinct meanings depending on the model. On most current Ferroli boilers, F34 signals that the electrical supply voltage has dropped below the 170V threshold the boiler needs to operate safely — essentially the control board has detected an undervoltage or irregular mains frequency condition. On certain older Ferroli Comby and i-series models, F34 is instead reported as a flame failure or ignition fault, where the boiler cannot establish or detect a stable flame. If you own an i-series unit (for example the i29), be aware that even when the mains supply measures a healthy 240V at the wall, a failing PCB can falsely trigger the low-voltage version of this code — so always confirm which definition applies to your specific model before proceeding.
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
- Genuine mains undervoltage or supply fluctuation Common
The UK mains supply should sit around 230–240V. During local grid problems, overloaded circuits, or faulty incoming supply cables, voltage can dip below the boiler's 170V minimum and trigger F34. Check whether other high-draw appliances in your home are also behaving erratically — this points to a supply issue rather than a boiler fault.
- Faulty or ageing PCB falsely detecting low voltage Common
Even when a multimeter confirms normal mains voltage at the boiler's terminal block, a degraded control board can misread the supply and throw F34. This is a well-documented complication on older Ferroli i-series boilers. The PCB's voltage-sensing circuit deteriorates over time, particularly in damp or high-temperature boiler cupboard environments.
- Loose or corroded wiring connections at the terminal block Sometimes
Internal wiring that has worked loose, corroded terminals, or damaged cable insulation between the consumer unit and the boiler can cause voltage to drop or become erratic at the board. The boiler may read a false undervoltage even though the supply at the spur socket is perfectly fine.
- Flame failure or ignition fault (older Comby and i-series models only) Sometimes
On certain legacy Ferroli models, F34 is mapped to an ignition or flame-detection failure rather than a voltage fault. Causes include a sooty or misaligned flame sensor electrode, insufficient gas pressure at the burner, or a failed ignition module preventing the spark from establishing a flame. Check your model's documentation or the boiler's display wording to confirm which definition applies.
How to fix it
- Check other appliances and your mains supply DIY safe
Before touching the boiler, look around your home. If lights are dimming, other appliances are tripping, or your smart meter shows unusual readings, the problem may lie with your incoming electricity supply. Contact your energy supplier or a registered electrician if you suspect a grid or incoming-cable issue.
- Attempt a single boiler reset DIY safe
Locate the reset button on your Ferroli's control panel — it is usually marked with an 'R' or a flame-with-arrow symbol. Press and hold it for one to two seconds until the boiler attempts to restart. If the fault clears and the boiler runs normally, monitor it over the next hour. Do not reset the boiler more than two or three times; repeated resets without a fix can mask a more serious underlying problem.
- Have an electrician verify the mains voltage at the boiler Gas Safe engineer
A registered electrician or Gas Safe engineer should use a calibrated multimeter to measure the voltage directly at the boiler's terminal block. If it reads around 240V, the mains supply is not the cause and the fault lies within the boiler itself — most likely the PCB's voltage-sensing circuit. If it reads significantly below 230V, the supply cable, spur fuse, or consumer unit connection needs attention by the electrician before the boiler fault can be resolved.
- Have a Gas Safe engineer inspect the wiring harness and internal connections Gas Safe engineer
If the external supply is confirmed healthy, an engineer should inspect the internal wiring loom for loose terminals, corrosion, or damaged insulation between the PCB and the terminal block. A small wiring fault caught early can be repaired at relatively low cost compared with a full PCB replacement.
- PCB testing and replacement if required Gas Safe engineer
If wiring is intact and the supply voltage is normal, the most likely remaining cause is a faulty PCB. A Gas Safe engineer will carry out electrical continuity tests on the control board. If the board is confirmed faulty, it will need to be replaced with a compatible Ferroli PCB — a job that must be carried out by a qualified engineer. On older or discontinued Ferroli models, sourcing the correct board can involve lead times, so ask your engineer to confirm part availability before committing to the repair.
- If F34 appears as an ignition fault: have the electrode and flame sensor inspected Gas Safe engineer
On models where F34 indicates flame failure, a Gas Safe engineer should inspect the ignition electrode for carbon deposits, cracking, or misalignment, and clean or replace it as needed. The flame detection sensor (ionisation probe) should also be checked. Gas pressure at the burner will be verified with a manometer. If the ignition module or gas valve is suspected, those components must only be handled by a Gas Safe registered engineer.
- Call a Gas Safe registered engineer if the fault persists or keeps returning Gas Safe engineer
If F34 reappears after a reset, or if you are unsure which variant of the fault applies to your model, book a visit from a Gas Safe registered engineer. They can definitively diagnose whether the fault is electrical, PCB-related, or ignition-related and carry out the appropriate repair safely. You can find engineers on the official Gas Safe Register at gassaferegister.co.uk.
Parts you may need
- Ferroli replacement PCB (model-specific) · from £280
- Ignition electrode · from £35
- Flame sensor / ionisation probe · from £30
- Wiring harness loom · from £55
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–£400, depending on the underlying cause.
Frequently asked questions
My Ferroli shows F34 but my voltage is fine — why?
This is a known issue on several Ferroli i-series models, including the i29. The boiler's PCB contains a dedicated voltage-sensing circuit that can deteriorate with age. Even when the mains supply measures a healthy 230–240V at the terminal block, a failing PCB can misread it and throw F34. In this situation the mains supply is not the problem — the control board itself needs to be tested and most likely replaced by a Gas Safe engineer. A PCB replacement on a Ferroli typically costs £200–£400 including parts and labour, though parts for discontinued models can push costs higher.
Is F34 the same on all Ferroli boilers?
No — this is an important distinction. On most modern Ferroli boilers, F34 means the supply voltage has fallen below 170V or the mains frequency is irregular. However, on certain older Ferroli Comby and i-series models, F34 is used to indicate a flame failure or ignition fault instead. The wording on your boiler's display (if it shows a description alongside the code) is the best guide, as is your model's installation manual. If you are unsure, a Gas Safe engineer can confirm the correct interpretation for your specific unit.
Can I fix a Ferroli F34 fault myself?
There is a limited amount a homeowner can safely do: check whether other appliances are affected by a supply issue, attempt one or two resets using the boiler's reset button, and call your energy supplier if you suspect a mains problem. Beyond that, anything involving the internal wiring, the PCB, the ignition electrode, or gas-carrying components must be handled by a Gas Safe registered engineer. Attempting DIY repairs on these parts is both dangerous and illegal in the UK.
Should I repair my Ferroli or replace it if F34 is a PCB fault?
If your Ferroli is under eight to ten years old and otherwise in good condition, a PCB repair or replacement is usually worthwhile — expect to pay roughly £200–£400 all-in. If the boiler is older than ten years, has had repeated faults, or the repair cost is approaching 30–40% of a new boiler's installed price (typically £2,000–£3,500 for a mid-range combi), replacement is often the more financially sensible decision. An annual Gas Safe service also significantly reduces the risk of PCB-related faults developing in the first place.