Boiler fault codes: how to read them and what to do next

A boiler fault code is the boiler telling you why it has shut itself down. The same code means different things on different makes, so read it against the manual for that exact model. Manufacturers let a householder make two checks: the system pressure, normally 1 to 1.5 bar, and whether gas is reaching the boiler (pre-payment credit, other gas appliances). Anything inside the casing is for a Gas Safe registered engineer. If you smell gas, call 0800 111 999.

This page shows how to read a code, three real examples from manufacturers' own pages, and where the line sits between a safe check and gas work.

What are boiler fault codes?

When a boiler detects a fault, it stops itself as a safety measure. Vaillant calls this a "lockout". The display shows a code, and the manufacturer's manual says what that code means and what to check.

A code is not a diagnosis of the part that has failed. Vaillant's own list gives several possible causes for one code, from a frozen condensate pipe to a faulty ignition electrode. The code tells you where to start looking.

Do boiler fault codes mean the same on every make?

No, and this is the mistake that costs return visits. The same letter and number can mean a different thing on another make, and the same fault has a different code on each. Here is one symptom on three boilers, as the manufacturers describe it.

Sources: Vaillant's fault code list, Ideal's fault code FAQ and the Greenstar 8000 Life manual (6720883865, edition 2021/11), all read on 28 September 2026. Other models from the same makers can use other codes, so treat this as an illustration, not a lookup table.

Where do you find Worcester Bosch and Vaillant fault codes?

Start with the model, not the code. Manuals are written per model and per edition. The Worcester Bosch manual above is titled "Greenstar 8000 Life", reference 6720883865, edition 2021/11, and its chapter 10 holds the fault code table with a remedy against each code.

• Vaillant publishes an overview list of fault codes on its site, and says its official list is only available from Vaillant.
• Worcester Bosch publishes an explanation of its display symbols and a troubleshooting tool, and each model's code table sits in that model's manual in its literature library.
• Ideal publishes a fault code FAQ, plus a troubleshooting guide for each range.

Worcester Bosch's manual notes that its fault finding information is for guidance only, and that it cannot be held responsible for costs incurred by persons not deemed to be competent.

Which fault code checks can a householder make?

Only the ones the manufacturer lists for the householder. From the pages above:

• System pressure. If the gauge is below the normal range, which Vaillant and Ideal both give as 1 to 1.5 bar, the boiler can lock out. Vaillant says that if the pressure is under 1 bar it can be topped up without an engineer if you are competent to do so, using the filling loop and at your own risk. Ideal says F1 means the pressure is too low and needs topping up to between 1 and 1.5 bar, and links a video showing how.
• Gas supply. Vaillant and Ideal both say to check that other gas appliances work and, on a pre-payment meter, that there is credit. Ideal also says to check the yellow gas isolation valve under the boiler has not been turned off.
• Frozen condensate pipe. In cold weather, an outdoor condensate pipe can freeze and stop the boiler firing. Vaillant says to pour warm water along the pipe, not boiling water from a kettle, which can crack it.
• A single reset, once the cause is dealt with. Vaillant says to resolve the cause first, and that if you find you need to reset the boiler regularly, that is not normal and you should get an installer to look. Ideal's list includes a code for a boiler reset too many times in 15 minutes.

Vaillant's fault code list is blunt about the rest: don't try to solve it yourself, as opening the boiler casing can be dangerous. Reset instructions vary by model, so use the manual.

What is only for a Gas Safe registered engineer?

Anything on the boiler itself, anything inside the casing, and any gas fault. Regulation 3 of the Gas Safety (Installation and Use) Regulations 1998 says nobody may work on a gas fitting unless they are competent, and that a self-employed person may only do that work if they are in a class of persons approved by HSE. In practice that is Gas Safe registration.

HSE also advises checking that the engineer is registered for the type of gas work involved, which is marked on the back of the Gas Safe Register card.

Two rules from HSE apply to anyone. It is illegal to use a gas appliance if you suspect it is unsafe. Turn it off and leave it until a Gas Safe registered engineer has checked it. If you suspect a gas leak, call the Gas Emergency freephone number on 0800 111 999, open all doors and windows, and shut the gas off at the meter control valve if you know where it is.

What should an engineer check first?

Before the code, the basics. Worcester Bosch's Greenstar 8000 Life manual lists the checks to make before fault finding: mains power and polarity, appliance settings, programmer, room thermostat, cylinder thermostat if fitted, cold water mains, system pressure, and gas to the appliance. That last one includes asking whether other gas appliances work and whether a pre-payment meter has run out of credit.

Then read the code against the manual for that model and work through the remedy steps in order. Its code 227, no flame signal after ignition, starts with the gas supply to the property and the inlet on the gas valve, and goes on to the ignition and condensate trap.

How should you log a fault call?

The next engineer, or the same one in a year, will be glad of it. Here is a short record to fill in on every fault visit. It is our own suggestion, not a manufacturer's form.

1. Customer and address
2. Boiler make, model and the manual edition you used
3. Fault code shown, and what the customer saw and heard
4. What you checked, and what you found
5. What you did, and any parts fitted
6. Result after the repair, and whether the code returned
7. What the customer was told, and any follow-up needed

If you also run landlord work, the same discipline applies to the annual check. See our guide to the CP12 gas safety record.

Where Nexley fits

Nexley is practical AI for UK trades that works through WhatsApp. It does not repair boilers, and gas work stays with the Gas Safe registered engineer.

What it can do is the lookup. Nexley holds boiler manuals, so you send the code and the model from the loft in a message or voice note, and it gives you the manufacturer's own check sequence for that code, from the manufacturer's document rather than a forum post. Then it can write up the call for you: code, cause, action, parts. You still make the call on site.

See the back office page, how AI works for plumbers, or try it free for 7 days, no card needed.

Common questions

What does F28 mean on a Vaillant boiler?

Vaillant's official explanation is that the flame signal was not detected during the ignition phase. Causes it lists include a frozen condensate pipe, low gas pressure, a faulty ignition electrode or circuit board, and a flue problem. If it is not a frozen pipe or pre-payment credit, Vaillant says it needs an engineer.

What does L2 mean on an Ideal boiler?

Ideal lists L2 as flame loss or ignition lockout. Its guidance is to reset the boiler, check pre-payment credit, check other gas appliances, and check the isolation valve. If that does not fix it, it says to call Ideal quoting the code. It adds that a gurgling noise while the boiler tries to fire means contact your installer, as there may be a blockage. If other gas appliances in the home also fail, contact a local gas engineer.

What does F22 mean on a Vaillant boiler?

Low water pressure in the system. Vaillant says pressure below the normal range of 1 to 1.5 bar can be topped up by a competent person, at their own risk. If you have found a leak, Vaillant says that is for a Gas Safe registered engineer.

Can I reset a boiler myself?

Manufacturers allow a reset once the cause is sorted, using the reset button as the manual describes. Vaillant's advice is that needing to reset a boiler regularly is not normal and should be looked at by an installer.

Who can fix a boiler fault code?

For anything on the boiler itself or inside the casing, a Gas Safe registered engineer. Householders should only do the checks the manufacturer lists.

Sources

This page is general guidance, not a repair manual. Fault codes vary by make and model. Always use the manual for the exact boiler.

• Vaillant, Boiler fault codes list (vaillant.co.uk)
• Vaillant, Understanding an F28 fault code (vaillant.co.uk)
• Vaillant, How to reset a boiler (vaillant.co.uk)
• Ideal Heating, What does my boiler fault code mean (idealheating.com)
• Worcester Bosch, Greenstar 8000 Life manual, 6720883865 (2021/11) (worcester-bosch.co.uk)
• Worcester Bosch, What do the error symbols mean on my boiler (worcester-bosch.co.uk)
• Worcester Bosch, Manuals and brochures (worcester-bosch.co.uk)
• HSE, Gas safety for landlords and letting agents (FAQ, unsafe appliances and gas leaks) (hse.gov.uk)
• HSE, Gas safety check: who can do it (hse.gov.uk)
• legislation.gov.uk, Gas Safety (Installation and Use) Regulations 1998, regulation 3 (legislation.gov.uk)