The "lockout" is a safety feature: the boiler stops because a sensor has detected something out of place. Sometimes it’s trivial (low pressure), sometimes not. Here are the checks you can safely make — and the limit beyond which insisting is counterproductive.
What to do, in order
The display shows a code (e.g. E01, F28): photograph it and look it up in the manual. It tells you straight away whether it’s an ignition, pressure, flue or sensor problem — and it will be very useful to the technician over the phone.
The gauge should read 1-1.5 bar when cold. If it’s below that, slowly open the filling valve up to 1.2-1.5 bar, close it firmly and reset the boiler with the reset button. If the pressure drops again over hours or days, there’s a leak in the circuit: you need a technician.
Gas valve open, power on (try resetting the breaker), room thermostat with fresh batteries and set to "winter". A reset (max two attempts) after these checks fixes a good share of lockouts.
What NOT to do
- Don’t reset more than twice in a row: if the lockout returns, the cause is real and repeated resets can make it worse
- NEVER open the boiler casing: inside there’s gas, a flame and 230V — it’s also against the law
- If you smell gas: no resets, no switches. Turn off the gas, open the windows and call the distributor’s emergency number from outside
- Don’t "bleed" the radiators as a cure for the lockout: it only matters if the problem is air in the system
When you need a technician
If the lockout returns after the reset, if the pressure keeps dropping, or if the code points to flue/fan/sensors: you need an instrumented diagnosis. With the error code already in hand, the technician often arrives with the right spare part and finishes in one visit.
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Frequently asked questions
The boiler only locks out when I ask for hot water: why?
Typical of a domestic hot water heat exchanger clogged with limescale, or of a sensor: heating works but the hot water side trips the protection. It’s a targeted repair — there’s no need to replace the boiler.
How often should maintenance be done to avoid lockouts?
Servicing as per the manual (usually annual) and a flue-gas check every 2 years for condensing boilers under 35 kW. Most winter lockouts come from skipped servicing.