Glossary · Faults, servicing and gas safety
What are boiler fault codes?
A boiler fault code is a letter and number combination shown on the boiler display when it detects a problem and shuts down or limits itself. The code points to the area of the fault, such as low water pressure or failed ignition. The same code can mean different things on different brands, so use the maker's own list.
Key points
- Fault codes are brand and model specific. Use the list for your own boiler, from the manufacturer's manual or website.
- Several common codes point to low water pressure, failed ignition or a frozen condensate pipe, and manufacturers describe a few safe checks for those.
- Vaillant and Glow-worm both say not to open the boiler casing, because it can be dangerous.
- Most codes on the manufacturers' lists say to call an engineer, and a code that keeps coming back needs one.
- If you smell gas, leave the property and call the National Gas Emergency Service on 0800 111 999.
How boiler fault codes work
A modern boiler monitors itself. It checks temperature sensors, water pressure, the pump, the fan and the flame. If a reading is outside its limits, the boiler can stop or run in a restricted way and show a code on the display. Baxi says that if its boiler develops a problem, the boiler will display an error code to help you identify the issue.[3]Baxi, Error codes
A code is a pointer, not a full diagnosis. Vaillant’s list gives, for each code, a meaning and a set of possible causes. Many of those causes are inside the boiler, such as a faulty sensor or circuit board, which only an engineer can test.[1]Vaillant, Boiler fault codes
The codes are not standard. Each manufacturer has its own list, and the list can vary by model. Always use the manufacturer’s own page or manual for your boiler. The examples below come from Vaillant, Glow-worm, Baxi and Worcester Bosch.
Examples from four manufacturers
The table shows a few common codes and what each manufacturer’s own page says they mean. It is not a full list.
| Brand | Code | Meaning on the manufacturer’s page |
|---|---|---|
| Vaillant | F.22 | Low water pressure in the system. Normal range is 1 to 1.5 bar |
| Vaillant | F.28 | The flame signal was not detected during the ignition phase |
| Vaillant | F.20 | The safety thermal cut out has operated and shut the boiler off |
| Glow-worm | F.22 | Low water pressure in the system |
| Glow-worm | F.28 | The boiler has failed to ignite |
| Baxi | E133 | Gas supply fault |
| Baxi | E118 | Water pressure low, under 0.5 bar |
| Baxi | E125 | Water circulation fault |
| Worcester Bosch | EA | Shown when the boiler repeatedly locks out. Worcester links it to a blocked condensate pipe |
These entries come from the manufacturers’ pages.[1]Vaillant, Boiler fault codes[2]Glow-worm, Boiler fault codes[3]Baxi, Error codes[4]Worcester Bosch, How do I know that my condensate pipe is blocked? For detail, see our entries on F22, F28, F75, EA and E133. A boiler that has shut itself down is often in lockout.
What you can check yourself
Manufacturers describe a few checks that a householder can do. Nothing else is safe to attempt.
Pressure. For low pressure, Vaillant says that if the gauge shows under 1 bar, you can raise it using the filling loop to between 1 and 1.5 bar if you are competent to do so. Vaillant says its guidance is used at your own risk, so use your user manual.[1]Vaillant, Boiler fault codes
Gas supply. For failed ignition, Vaillant and Glow-worm say to check that other gas appliances in the home work and, with a prepayment meter, that there is credit.[1]Vaillant, Boiler fault codes[2]Glow-worm, Boiler fault codes
Frozen condensate pipe. In cold weather, check for ice in an outdoor condensate pipe. Vaillant and Glow-worm point to a video on thawing it.[1]Vaillant, Boiler fault codes[2]Glow-worm, Boiler fault codes Worcester Bosch says that, if it is safe to do so, you can pour warm water, not boiling water, along the pipe and then reset the boiler.[6]Worcester Bosch, Boiler not working in freezing temperatures?
Reset. Vaillant and Glow-worm both say a reset can clear a fault or get the boiler working again for a while. Follow the manual.[1]Vaillant, Boiler fault codes
When to call an engineer
Vaillant and Glow-worm both advise against opening the boiler casing, because it can be dangerous. Most codes on their lists say to contact an engineer.[1]Vaillant, Boiler fault codes[2]Glow-worm, Boiler fault codes By law, work on a gas appliance must be done by a Gas Safe registered engineer.[5]Health and Safety Executive, Domestic gas: frequently asked questions If you smell gas, leave the property and call the National Gas Emergency Service on 0800 111 999.[1]Vaillant, Boiler fault codes If the same code comes back after a reset, stop resetting and book a boiler repair.
Example
On a frosty January morning, Tom in Keighley finds his heating off and a code on the display. The pressure gauge shows 1.2 bar, so it is not low pressure. He checks the manufacturer’s list, sees that the code is about ignition, and finds the outside condensate pipe is frozen. He thaws it with warm water from a watering can, resets the boiler once as his manual describes, and the heating restarts. If it had not, he would have called an engineer rather than open the casing.
What people get wrong
Many people search a code on the internet and follow the first fix they find. Codes differ between brands and models, and the fix for one may be wrong or unsafe for another. Use your own manufacturer’s list, and stop when it says “this is a case for an engineer”.
The second mistake is resetting again and again. A reset can clear a fault for a while, but a code that returns means something underlying is wrong.[1]Vaillant, Boiler fault codes
The bottom line
A fault code tells you where to look, not what to do. Find the code in your own manufacturer’s list, do only the checks it describes and leave the boiler casing closed. If the code is not on a short list of safe checks, or it returns, call a Gas Safe registered engineer.
Common questions
What does a boiler fault code mean?
It tells you which part of the boiler or heating system the control board has flagged, for example a sensor, the water pressure or the ignition. The exact meaning depends on the make and model, so check the manufacturer's list.
Can I reset my boiler when it shows a fault code?
Vaillant and Glow-worm both say resetting can resolve a fault or get the boiler working again for a while. Follow your manual for the right way to reset. If the code returns, call an engineer.
Which fault codes can I deal with myself?
Only the checks the manufacturer describes, such as topping up low pressure through the filling loop, checking prepayment meter credit or thawing a frozen condensate pipe with warm water. Everything else is for an engineer.
Are fault codes the same on every boiler?
No. Vaillant and Glow-worm use F.28 for a failed ignition. Baxi uses E133 and Worcester Bosch uses EA. Even within one brand, a code can differ between models.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Vaillant. Boiler fault codes. Checked 11 November 2026. ↩
- Glow-worm. Boiler fault codes. Checked 11 November 2026. ↩
- Baxi. Error codes. Checked 11 November 2026. ↩
- Worcester Bosch. How do I know that my condensate pipe is blocked?. Checked 11 November 2026. ↩
- Health and Safety Executive. Domestic gas: frequently asked questions. Checked 11 November 2026. ↩
- Worcester Bosch. Boiler not working in freezing temperatures?. Checked 11 November 2026. ↩