Smell gas? Turn off the gas at the meter, open windows, do not use switches and call the National Gas Emergency Service on 0800 111 999.Smell gas? Call 0800 111 999

UK boiler fault code library

Boiler fault codes explained, in plain English

Type the code on your boiler display to see what it means, the safe checks you can do yourself, and when you need a Gas Safe engineer.

Popular: Vaillant F22Vaillant F28Vaillant F29Vaillant F75Glow-worm F22Glow-worm F28
  • 8 boiler brands
  • 52 fault codes explained
  • Safety first advice on every page

Choose your boiler brand

All brands →
VA Vaillant 12 fault codes ID Ideal 10 fault codes WB Worcester Bosch 8 fault codes BA Baxi 8 fault codes GL Glow-worm 4 fault codes VI Viessmann 4 fault codes VO Vokera 3 fault codes PO Potterton 3 fault codes

Most searched fault codes

VaillantDIY fix F22 Low water pressure (dry fire protection) VaillantSafe checks F28 Ignition failure on start-up VaillantSafe checks F29 Flame loss during operation VaillantEngineer F75 No pressure rise detected when pump starts Glow-wormDIY fix F22 Low water pressure in the boiler Glow-wormSafe checks F28 Ignition failure when starting up Glow-wormEngineer F75 Pressure sensor fault (no rise when pump starts) IdealSafe checks L2 Ignition lockout: boiler failed to light IdealDIY fix F1 Low water pressure IdealSafe checks F2 Flame loss during operation VokeraSafe checks A01 Ignition failure: flame not sensed VokeraDIY fix A04 Insufficient system water pressure

How to read our advice

DIY fix

Something most homeowners can sort safely, like topping up pressure or resetting the boiler.

Safe checks

Try the simple checks we list first. If the code comes back, book an engineer.

Engineer

An internal fault. Only a Gas Safe registered engineer should open the boiler.

Boiler troubleshooting guides

All guides →
Boiler Pressure Too High? Causes and How to Lower It Safely Why boiler pressure climbs too high, how to bring it down by bleeding a radiator and when to call an engineer 5 min read Boiler Pressure Too Low? How to Repressurise a Combi Boiler Top up a combi with the filling loop to 1 to 1.5 bar and find out why the pressure keeps dropping 6 min read Boiler Repair Cost in the UK: Typical Prices by Part Typical UK repair prices for common boiler parts and the fault codes that point to each one 5 min read Frozen Condensate Pipe: How to Spot It, Thaw It and Stop It Spot a frozen condensate pipe, thaw it safely with warm water and stop it freezing again 5 min read New Boiler Cost in the UK: Combi, System and Regular Prices Typical UK supply and fit prices for combi, system and regular boilers, and what drives the cost 5 min read Boiler Leaking Water: Causes, What to Do Now and Repair Costs What to do when a boiler leaks, where the water comes from and what the repair is likely to cost 4 min read

Why your boiler is showing a fault code

Modern condensing boilers monitor themselves constantly. When a sensor reads something outside its safe range, such as low water pressure, a flame that will not light or a pump that is not moving water, the boiler shuts down and shows a code on its display. The code is the boiler telling you what it detected, so it is the fastest route to the right fix.

Every manufacturer uses its own system. A Vaillant shows codes like F22 or F28, an Ideal shows L2 or F1, a Worcester Bosch uses combinations such as EA 227, and a Baxi uses E-codes like E133. The same letters can mean different things on different brands, which is why this site organises every code by manufacturer first.

What you can safely do yourself

A large share of the codes UK homeowners see each winter come down to three simple causes: pressure that has dropped below 1 bar, a condensate pipe that has frozen in cold weather, or an interruption to the gas supply such as an empty prepayment meter. All three can usually be sorted without tools and without opening the boiler. Each fault code page on this site lists the checks that are safe for a homeowner, in the order we would try them.

When you need a Gas Safe engineer

Anything inside the boiler casing is off limits. Faults with the gas valve, fan, printed circuit board, ignition electrodes, heat exchanger or flue must be handled by a Gas Safe registered engineer. It is a legal requirement in the UK, and it protects you from carbon monoxide and gas leaks. Where a code points to an internal fault we say so clearly, and we give typical UK repair costs so you know what to expect before you book.