Fault symbol (warning, ‘!’, or a code on the display)
The boiler has detected a fault. It nearly always comes with a code that only the maker’s own table explains.
What the manual says to do
Note the code and what the boiler was doing. Check the pressure and the gas supply as the manual lists. If the manual marks the fault as one the user may reset, reset once; otherwise, or if it returns, contact your installer or the maker with the code. Worcester Bosch asks for the displayed fault code, the appliance type and serial number, and the commissioning date.
Only your boiler’s own manual says exactly what a symbol means on your model. The manuals let the householder reset a fault once, top up the pressure through the filling loop and change the settings; anything else on a gas boiler is a job for a qualified heating engineer (Gas Safe registered in the UK), as the manuals themselves say.
What it does not mean
The symbol does not tell you what is wrong by itself, and it does not mean the boiler is dangerous to stand near. This site does not decode the codes: they differ by maker and model.
- What it looks like
- A warning mark or ‘!’ next to a code: F. and two digits on Vaillant, a number such as 214 on Worcester Bosch, E and a number on Baxi, F or L and a number on Ideal.
- Names used
- Fault indicator (Worcester Bosch); Fault in the product, F.XX (Vaillant); Generic error (Baxi); Fault display active (Viessmann); F or L code (Ideal)
Where makers differ
Baxi separates a generic error, errors that stop the burner starting, a resettable error and an error the user cannot reset (the spanner). Viessmann can list up to 5 fault messages. Vaillant shows the fault instead of the basic display, sometimes with plain text.
Other symbols with the same shapes
Sources
No public standard defines this boiler symbol, so its meaning is taken from the manuals of 4 different maker groups (Worcester Bosch, Vaillant, Baxi, Viessmann), each fetched and read on the date shown:
- Worcester Bosch, Greenstar 2000 combi (GR2301iW): Greenstar 2000 user instructions, 6 721 839 343 (2023/04) UK/IE (PDF) — Screen display: “Fault indicator — Displays during a fault condition”; the cause is displayed in code (e.g. fault code 214); give the installer the fault code, appliance type and serial number, and commissioning date. Checked 2026-09-27.
- Vaillant, ecoTEC plus VU, VUW, VUI ..6/5-5: Operating instructions ecoTEC plus, 0020245033_02 (26.05.2021) GB, IE (PDF) — “Fault in the product — Appears instead of the basic display, may be an explanatory plain text display. F.XX”; if fault messages (F.xx) appear, consult a competent person. Checked 2026-09-27.
- Baxi, Baxi Compact range (quick guide 7849761): Quick start guide to your boiler, 7849761 Baxi Compact quick guide user instructions (PDF) — Display symbols: “Generic error”, “Errors that prevent the burner from starting”, “Indicates a resettable error”, “Error – not resettable by user”. Checked 2026-09-27.
- Viessmann, Vitodens 100-W/111-W/111-F/141-F (black/white screen): Operating instructions for the system user, 6167584 GB (12/2020) (PDF) — Symbols: “Fault display active”; “Up to 5 fault messages can be displayed.” Checked 2026-09-27.
Written in our own words; quotations are the makers’ wording.