Boiler fault codes explained

Boiler fault codes are short messages, usually a letter and a number, that a boiler shows on its display when something has stopped it working properly. The code is the manufacturer’s own, so the same number can mean different things on a Vaillant, an Ideal and a Worcester. Find the code on your display, look it up for your make and model, and do only the checks that the manufacturer says are safe for a homeowner.

How to read a boiler fault code

Start with what the display shows. Most boilers show a letter and a number, such as F22, and some show a flashing light or a sequence instead. Write down exactly what you see, and photograph it if it flashes. Add the make and model from the front or the data plate, because an engineer will ask for those first. Keep a note of each code and its date too, because a pattern helps the engineer.

Then use the manufacturer’s manual. Vaillant, for one, doesn’t explain its display on its fault code page. It sends you to the manual, which it publishes through a literature finder on its website. The manual also tells you how to reset the boiler, which is not the same on every model.

A code is a clue, not a diagnosis. It says which part of the boiler noticed the problem, and not always why. A low pressure code, for example, tells you the water has dropped. It does not tell you whether you topped it up too late or whether there is a leak.

Boiler fault code checks you can make yourself

Vaillant’s guidance is a good example of what a manufacturer accepts from a homeowner, and it is short. For the flame and ignition codes it asks you to check three things before you call anyone:

• Other gas appliances. If your hob or fire doesn’t work either, the problem is the gas supply and not the boiler.
• Pre-payment credit. If you have a pre-payment gas meter, top it up and see whether the code clears.
• A frozen condensate pipe. In cold weather, check the outdoor condensate pipe for ice. A frozen pipe is a common winter cause of a boiler shutting down.

For a pressure code the check is the gauge and the filling loop, as our guide to Vaillant F22 describes. Vaillant’s advice is to bring the pressure back into the normal range of 1 to 1.5 bar slowly, and to follow the manual for the details.

Resetting is the other thing most makers allow. Vaillant says that a reset often clears a fault or gets the boiler going again for a while. It also says clearly that you should not open the boiler casing, because that is dangerous.

Fault codes that are an engineer’s job

Some codes are not worth your time. They point to a part inside the boiler, or to a safety device, and the manufacturer says to call an installer or engineer. Taking Vaillant’s published list as an example, these are the kinds of fault that fall outside the homeowner’s checks:

Code	What it means in plain terms	What the maker says
F.0, F.1	A flow or return temperature sensor has stopped sending a signal	Call an engineer
F.20	The safety cut-out has shut the boiler off	Check pressure, then the installer
F.27	The boiler senses a flame when it shouldn’t	Don’t attempt it yourself
F.32	The fan has a fault	Call an engineer
F.35	The air or flue pipe is blocked	Installer first

A blocked flue or a flame where there should be none is a safety matter. Those are the codes where a keen amateur can do the most harm. If a code isn’t on the manufacturer’s list at all, ring their technical line and don’t guess.

When a boiler fault code needs a Gas Safe engineer

Call a Gas Safe registered engineer if the checks above don’t clear the code, if the same code returns within days, or if the code is one that the manufacturer reserves for an engineer. Gas work, including work on a boiler, has to be done by a registered engineer, so ask to see the Gas Safe ID card before they start, and check that it lists the type of work they are doing.

Call the National Gas Emergency Service on 0800 111 999 if you smell gas. Leave the property first, and don’t use switches, a lighter or a phone on the way out of the room that smells.

If the boiler is still under guarantee, contact the manufacturer first. A visit from a different engineer can affect the cover.

Which makes and codes this guide covers

The guides in this section follow the codes people search for most, make by make, and each one links back here for the general rules. For Vaillant, F22 is the low pressure fault and F28 is a flame that wasn’t detected. For Ideal, F1 is low pressure and L2 is a flame loss lockout. Codes are manufacturer specific, so a Vaillant guide won’t help you read an Ideal, and we say which make each guide is for. Your own manual lists your boiler’s codes. Check it before you rely on a number from another make.

When faults point to a new boiler

One fault code is routine. A boiler that shows a different one every few weeks, that is getting hard to repair or that costs more to fix than you want to spend is telling you something else. Older boilers are also less efficient than a modern condensing one, so a replacement can lower your bills as well as end the faults.

A like-for-like combi boiler swap is the cheapest replacement, and the combi boiler prices page shows what each type and model costs fitted, including VAT. Our guide to boiler grants explains what help exists, which is less than most people expect for a gas boiler.

When you ask for quotes, use the compare tool to line them up, and the quote checker to see what each one leaves out. You can also get free quotes from Gas Safe installers near you.

Sources: Vaillant, boiler fault codes, read on 3 November. Manufacturers’ guidance is used at your own risk, and always check your own manual for your model.