BoilerFaultCodesKey findings
of boiler fault codes require a Gas Safe registered engineer — only 22% are safe for a homeowner to fix.
average boiler repair cost, ranging from £8 to £1,200 for the most serious faults.
is the single most common fault code, appearing on 176 different boiler models.
(control board) is the part most often implicated in boiler faults — ahead of the gas valve and circulation pump.
How we compiled this
Every figure is computed directly from our fault-code database — the largest in the UK — which is sourced from manufacturers' official service and installation manuals. We de-duplicate to distinct (brand, model, code) combinations so shared codes don't inflate the totals. Repair costs use the midpoint of each fault's typical UK price range (parts + labour). "DIY-safe" reflects our conservative classification — a fault is only flagged DIY-safe when it involves no gas, electrical, flue or sealed-component work. Full method on our sourcing page.
Only 1 in 4 boiler faults is safe to fix yourself
Of the 51,919 fault codes we analysed, just 22% can be safely resolved by a homeowner — things like topping up pressure, thawing a frozen condensate pipe, or resetting after a power cut. The remaining 78% involve gas, electrical or sealed components and legally require a Gas Safe registered engineer.
The 10 most common UK boiler fault codes
Ranked by how many distinct boiler models display each code. The generic E-series codes dominate because so many brands use the same numbering scheme.
Number of boiler models each code appears on.
Which boiler brands have the most fault codes?
Important: this measures the breadth of a brand's diagnostic system — how many distinct fault codes its boilers can display — not reliability or how often they break down. A boiler with more fault codes simply reports problems in more detail. More codes is not "worse".
Distinct fault codes documented across each brand's models.
The parts most often behind a boiler fault
Across all fault codes, these components are the ones most frequently named as the likely culprit or replacement.
Use this research
This data is free to cite and republish under a CC BY 4.0 licence — all we ask is a link back to this page so readers can see the full methodology. Journalists and editors: for custom cuts of the data (by brand, region, or fault type) email press@boilerfaultcodes.com.
Reviewed by the BoilerFaultCodes Editorial Team · Data last refreshed 31 August 2026. Figures are computed from manufacturer service manuals and may differ from individual real-world repair experiences.