Boiler keeps locking out: causes and fixes
A boiler that locks out cuts the heating and hot water as a precaution. Some causes take two minutes to fix (pressure, bleeding), others absolutely require a heating engineer.
Safety first
If you smell gas, do not operate any switch: shut off the gas supply, ventilate, and call the gas emergency number (0 800 47 33 33 in France). Never try to reset a boiler that smells of gas or burning.
The causes, from most to least common
Check them in this order: the first ones are free and solve the majority of cases.
- 1
System pressure too low
very commonHow to check: Read the pressure gauge: it should sit between 1 and 1.5 bar when cold. Below 0.8 bar, the boiler faults.
Fix: Repressurise using the filling valve to around 1.2 bar, then reset.
EasyEstimated cost: 0 € - 2
Air in the system
very commonHow to check: Radiators stay cold at the top and warm at the bottom, with gurgling noises.
Fix: Bleed the radiators one by one, starting with the one nearest the boiler, then repressurise.
EasyEstimated cost: 0 € - 3
Sludge in the system
commonHow to check: Faults keep repeating, radiators heat poorly despite correct pressure, and the water from bleeding is black.
Fix: Power-flush the system through a professional.
ProfessionalEstimated cost: 400 to 900 € - 4
Faulty sensor, valve or ignition electrode
commonHow to check: A specific error code shows and comes back every time after a reset.
Fix: Diagnosis and replacement by a heating engineer.
ProfessionalEstimated cost: 150 to 400 € - 5
Blocked flue
less commonHow to check: The boiler shuts down shortly after starting. A bird's nest or another blockage in the flue.
Fix: Professional work required - risk of carbon monoxide poisoning.
ProfessionalEstimated cost: 150 to 300 €
A boiler is not covered by the French repair bonus, which targets everyday electrical and electronic equipment. Its annual service is compulsory, though, and other grants exist to help replace it. See the list of covered appliances.
When to call a professional
If the boiler locks out again after two resets, stop and call a heating engineer: pushing on can make the fault worse. Any suspicion of gas, burning or a flue problem calls for an immediate professional.
Find a repairer near you →Frequently asked questions
How do you repressurise a boiler?
Find the filling valve (often a small lever under the boiler), open it gently while watching the gauge until it reaches 1 to 1.5 bar, then close it. Then reset the boiler with the reset button.
My boiler locks out every day, is that serious?
A fault that returns daily signals an underlying problem: sludge, a leak, a failing sensor or a blocked flue. Don't keep resetting it - get a heating engineer out quickly.
How many times can you reset a boiler?
Twice at most. If the fault comes back, the cause is real and persistent: repeated resets risk making it worse, or even creating a dangerous situation.
Your boiler may be nearing the end of its life: a boiler lasts 15 to 20 years. Beyond that, an expensive repair is harder to justify.