Glossary · Faults, servicing and gas safety
What is boiler lockout?
Boiler lockout is a safety feature. When the boiler detects a fault, such as low water pressure or ignition failure, it shuts itself down and stays off until the fault is dealt with and the boiler is reset. It protects the boiler and the people in the home from it running in an unsafe condition.
Key points
- Lockout is the boiler protecting itself and you. It is not a fault in its own right but a response to one.
- Worcester Bosch says the sign is usually a flashing light, a letter and number on a display or a written message.
- Manufacturers describe a reset button, dial or switch. The method differs by model, so check your user manual.
- If lockout keeps happening, Baxi and Vaillant both say a fault is indicated and an engineer is needed.
- Ignition, gas supply and overheating faults need a Gas Safe registered engineer before the boiler is reset.
How boiler lockout works
A boiler checks itself as it runs. Worcester Bosch describes lockout as a safety feature that stops the boiler operating under unsafe conditions. When a fault occurs, such as low water pressure or ignition failure, the boiler disables itself until the issue is resolved.[1]Worcester Bosch, What is boiler lockout and how do I fix it? Vaillant describes it the same way: the boiler shuts itself down as a safety precaution.[2]Vaillant, How to reset a boiler
The sign depends on the make. Worcester Bosch says it is generally a flashing light, sometimes a letter and number on a display, or a written message on a text display, and it advises checking the user manual for your boiler.[1]Worcester Bosch, What is boiler lockout and how do I fix it? Baxi boilers show a red lockout light, either constant or flashing.[3]Baxi, How to reset a Baxi boiler
Common reasons for lockout
Vaillant lists the following reasons.
| Reason | What Vaillant says |
|---|---|
| Ignition failure or gas supply | Common causes are blockages in the flue, a faulty gas valve or a problem with the ignition lead. A Gas Safe registered engineer needs to look at it before the boiler is reset. |
| Low pressure | The system has lost water. The pressure can be topped up following the manual, but a leak needs an engineer. |
| High pressure | Pressure above the normal range. Bleeding radiators can bring it down, and if that fails an engineer is needed. |
| Overheating | Blockages in the system or a failed pump can overheat the boiler. Vaillant says a qualified professional must deal with this. |
| Pay-as-you-go meter | If the gas has run out, the boiler may shut down. |
Baxi shows a fault code on one of its displays, E133, as a problem with either the gas supply, ignition or flame, or a blocked condensate pipe.[3]Baxi, How to reset a Baxi boiler There is more on codes on our boiler fault codes page.
Resetting
Resetting is not a repair. Vaillant says to resolve the cause first and says that, apart from very minor fixes such as defrosting a condensate pipe, bleeding radiators or topping up pressure, all errors should be looked at by a Gas Safe registered engineer.[2]Vaillant, How to reset a boiler
The reset itself varies by model. Baxi describes pressing a reset button until the red light goes out, turning a temperature knob fully anticlockwise, or holding a selector switch at R for at least 5 seconds.[3]Baxi, How to reset a Baxi boiler Vaillant says its reset button is held for about three seconds.[2]Vaillant, How to reset a boiler Follow your own user manual.
If the boiler does not restart, or locks out again, stop there. Vaillant says to contact a Gas Safe registered engineer if it does not turn on after resetting or the error code stays, and that regular resets should not be needed.[2]Vaillant, How to reset a boiler Baxi says frequent lockout indicates a fault.[3]Baxi, How to reset a Baxi boiler
If you smell gas, do not carry on with resets. HSE says to call National Grid’s Gas Emergency Freephone number, 0800 111 999, open all doors and windows and shut off the gas at the meter control valve if you know where it is. It also says to turn off an appliance you suspect is unsafe and not touch it until a Gas Safe registered engineer has checked it.[4]Health and Safety Executive, Domestic gas: frequently asked questions
Example
Gurpreet in Bradford finds her boiler off with a flashing light on a cold morning. She reads the manual, presses the reset button once and the boiler fires. Two days later it locks out again. She does not reset it a third time. She books a Gas Safe registered engineer, who finds the cause and fixes it.
What people get wrong
Many people treat the reset button as the repair. It only restarts the boiler. The boiler locked out because it detected a fault, and unless that is dealt with it will lock out again.
The second mistake is resetting again and again. Worcester Bosch says lockout can be a sign of deeper problems if it happens often.[1]Worcester Bosch, What is boiler lockout and how do I fix it?
The bottom line
Boiler lockout is a boiler switching itself off for safety after a fault. A single reset, done as your manual describes, is reasonable. A boiler that locks out again, shows an ignition or gas fault or overheats needs a Gas Safe registered engineer. If you smell gas, ring the National Gas Emergency number.
Common questions
Is a boiler lockout the same as a breakdown?
Not always. Worcester Bosch describes lockout as a safety feature that disables the boiler until the issue is resolved. The cause may be minor, but it can also be a sign of a serious fault, so the cause matters more than the reset.
How do I reset a locked out boiler?
It depends on the model. Baxi describes pressing a reset button, turning a knob or holding a selector switch at R. Vaillant says its reset button is held for about three seconds. Check your user manual.
How many times can I reset it?
Once is a reasonable test. Vaillant says you should not need to reset the boiler regularly, and Baxi says frequent lockout indicates a fault. Repeated lockouts are a reason to call an engineer.
What if I smell gas?
HSE says to call National Grid's Gas Emergency Freephone number, 0800 111 999, open all doors and windows and shut off the gas at the meter control valve if you know where it is. It also says not to touch a gas appliance you suspect is unsafe until a Gas Safe registered engineer has checked it.
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.
- Worcester Bosch. What is boiler lockout and how do I fix it?. Checked 11 November 2026. ↩
- Vaillant. How to reset a boiler. Checked 11 November 2026. ↩
- Baxi. How to reset a Baxi boiler. Checked 11 November 2026. ↩
- Health and Safety Executive. Domestic gas: frequently asked questions. Checked 11 November 2026. ↩