Boiler not working: the checks before you call an engineer

Safety first, then a few checks you can do yourself.

The answer in 2 lines. First make sure there is no smell of gas and no sign of carbon monoxide: if there is, get out and call for help. Otherwise, a few checks without tools often get a boiler going again: power and the boiler's switch, the thermostat and timer, the pressure gauge, a frozen condensate pipe in cold weather, and one reset if the manual allows it. If the fault comes back, call a Gas Safe registered engineer; if you rent, tell your landlord in writing.

The first 3 steps

Check there is no smell of gas and nobody has headaches, dizziness or nausea (see below). If in doubt, get out and call.
Note what you see: blank display, red light, fault code, the number on the pressure gauge, and whether it is freezing outside.
Do the checks below, once each. Do not keep resetting the boiler.

Stop: danger

Smell of gas: do not use switches, flames or your phone indoors; open doors and windows, turn off the gas at the meter if you can, get out and call the free National Gas Emergency Service on 0800 111 999 (England, Scotland and Wales), 24/7.
Headaches, dizziness, nausea or tiredness, especially in several people at once or easing when you go outside: think carbon monoxide. Open windows, switch the appliance off if you can, get out into fresh air and call 999 if anyone feels unwell (or NHS 111 for advice). Do not go back in until the appliance has been checked.
Never take the boiler casing off or touch the gas pipework. Only a Gas Safe registered engineer may work on gas appliances.
Black or sooty marks, a lazy yellow or orange flame, a burning smell or smoke: switch the boiler off, open windows and call a Gas Safe engineer. Do not restart it.
To keep warm meanwhile: never use a barbecue, camping stove, patio heater or generator indoors.

The checks, in order

Nothing lit on the boiler: are other lights and sockets working (power cut: call 105)? Is the boiler's switch (a fused spur on the wall nearby) on? Has a switch tripped at the consumer unit? If the consumer unit is dry and your hands are dry, reset it once; if it trips again, leave it and call an engineer or electrician.
Boiler on, radiators cold: check the thermostat (batteries, set above room temperature), the timer or programmer (heating not on "off" or holiday mode; the clock may have reset after a power cut), and that the radiator valves are open.
Pressure gauge low (usually below 1 bar when cold): many combi and system boilers stop. Top up only as your manual says, boiler off and cold; see our guide on boiler pressure.
Freezing weather, fault code, gurgling noise: on a condensing boiler, the plastic condensate pipe that runs outside may be frozen. Thaw it from the open end with warm (not boiling) water or a hot water bottle, then reset the boiler, as Worcester Bosch explains. Only do this on pipework you can reach from the ground.
Fault code or red light: look up the code in the manual (or the manufacturer's website using the model on the front). If the manual allows it, press reset once.
Gas hob not working either: on a prepayment meter, check your credit and emergency credit. Otherwise call 0800 111 999.

What you can do yourself, and where to stop

You can: check power, thermostat and timer; top up the pressure if the manual shows how; thaw a condensate pipe; reset once; bleed radiators that are cold at the top.
Stop there if it locks out again, the pressure keeps dropping, water is leaking from the boiler, it makes banging noises, or anything involves the gas, the flue or the inside of the boiler: call a Gas Safe registered engineer.

What not to do

Pressing reset again and again: the boiler is telling you something is wrong.
Pouring boiling water on a frozen condensate pipe: it can crack the plastic.
Blocking vents or air bricks to keep the cold out: the boiler and you need that air.

Who to call and who pays

Gas Safe registered engineer: A gas engineer must be on the Gas Safe Register (it is the law). Ask to see their Gas Safe ID card and check it on the register: how to check an engineer (HSE). Choose one whose card lists the work you need (for example "boilers" for a gas boiler). If the boiler is under warranty or you have boiler cover, call the manufacturer or your cover provider first.
If you rent: in England your landlord must keep the installations for heating and hot water in repair and working (Landlord and Tenant Act 1985, section 11), and must have gas appliances checked every year by a Gas Safe engineer and give you the record (HSE). Report the fault in writing (email or text is fine) and keep a copy. If nothing happens, contact your council's private housing (environmental health) team; Shelter England explains your options.
Council or housing association tenants: under Awaab's Law, in force since October 2025, social landlords must deal with emergency hazards within 24 hours (GOV.UK guidance for tenants). A second phase covering more hazards starts on 30 November 2026 (GOV.UK announcement).
Cannot afford a repair or replacement: Citizens Advice explains what help may be available.

Good to know

Have your boiler serviced once a year by a Gas Safe engineer, ideally before winter.
Keep the manual, or a photo of the model name and the pressure gauge when it is working normally, near the boiler.
Northern Ireland, Scotland and Wales have their own rules for rented homes; in Northern Ireland, see nidirect for gas safety.

On a frosty January morning in Manchester, Tom's combi boiler shows a fault code and gurgles. No smell, nobody feels ill. The manual says the code can mean a blocked condensate pipe; the plastic pipe outside is frozen at the end. He pours a few jugs of warm water along it, presses reset once, and the heating comes back. He lags the pipe that weekend.

A made-up story, for illustration.

Bobby

Sources, verification, confidence

Verified on 1 November 2026, for England. Confidence: high for the safety steps, the gas emergency number and the landlord duties in England (repairs, annual gas safety check, Awaab's Law dates for social housing); medium for typical pressure values and fault codes, which depend on your boiler (your manual comes first). No official statistics on boiler breakdowns are quoted. This guide describes common cases, not a diagnosis.