A calm, step-by-step guide to the safe checks you can make yourself when your boiler stops working — and a clear line on when it's time to call a Gas Safe registered engineer.
A boiler that won't fire up — no heating, no hot water, or both — is one of those problems that always seems to land on the coldest morning of the year. The good news is that a fair number of breakdowns come down to something simple you can safely check yourself in a few minutes. The rest need a qualified engineer. This guide walks you through the safe homeowner checks first, then tells you exactly where to stop.
Boilers need an electrical supply to run, so start there. Look at the display — is it lit at all? Then check:
It sounds obvious, but a surprising number of "broken" boilers are working perfectly — they just haven't been told to come on. Check your room thermostat is set above the current room temperature, and that the programmer or timer is calling for heat (not stuck on an "off" or holiday setting). If you have a smart thermostat, check it hasn't lost its connection or run its batteries flat. After a power cut, programmers often reset to a default time, so the schedule may simply be out of sync.
Most combi and system boilers have a pressure gauge — a round dial or a digital readout. When the system is cold, normal pressure is roughly 1 to 1.5 bar, rising towards 2 bar as it heats up. If the needle is sitting below about 1 bar, low pressure may be stopping the boiler from firing, and many boilers show a low-pressure fault code when this happens.
You can usually top this up yourself using the filling loop — the braided silver hose with a valve (or two) underneath the boiler. With the boiler off, open the valve(s) slowly and watch the gauge rise to around 1.5 bar, then close them firmly. If pressure keeps dropping again over days or weeks, there's a leak somewhere in the system, and that needs an engineer to trace and fix.
If the display shows a fault and the boiler has locked out, there's usually a reset button on the front panel. Press and hold it as the manual describes (often 3 seconds) and give the boiler a minute to attempt to restart. Reset it once. If it locks out again, don't keep resetting — a lockout is the boiler protecting itself, and repeatedly clearing it without fixing the cause is asking for trouble. Note the fault code on the display so you can give it to an engineer.
Modern boilers are good at telling you what's wrong. A code like F1, F22, E133 or similar points to a specific problem — low pressure, ignition failure, a faulty sensor, and so on. Codes differ by manufacturer, so look yours up in the manual or on the maker's website. Some codes (low pressure) you can act on; many (ignition or gas-related faults) are an engineer's job. Either way, writing down the exact code saves time and often money when you call out a professional.
In a cold snap, this is one of the most common winter breakdowns — and one of the few you can often fix yourself. Modern condensing boilers drain a small amount of acidic water through a plastic condensate pipe, and if part of that pipe runs outside, it can freeze and block. Tell-tale signs are a gurgling sound and a fault code or lockout that appeared overnight in freezing weather.
If you can safely reach the external pipe, you can thaw it by pouring warm (not boiling) water along it, or holding a covered hot-water bottle against the frozen section. Once it's clear, reset the boiler. Never use boiling water — it can crack the pipe — and don't attempt this if the pipe is high up or hard to reach safely.
If the boiler is getting power but won't fire, check whether your other gas appliances work — for example, does your gas hob light? If nothing gas-powered is working, the issue may be your gas supply rather than the boiler itself. If you're on a prepayment meter, check it's in credit. And, again, if you smell gas at any point, stop, ventilate, and call 0800 111 999.
If you've worked through the checks above and the boiler still won't run — or any of these apply — it's time to call a professional:
Crucially, all of these need the casing off or work on the gas circuit — that is strictly Gas Safe registered work. Always check your engineer's ID and registration on the Gas Safe Register before they start. (The old "CORGI" scheme stopped being the gas registration body back in 2009.)
Repair costs vary with the fault, the part and where you live, but the figures below give a rough idea. They're indicative and current to 2026 — get a written quote before any work goes ahead.
| Typical job | Indicative cost |
|---|---|
| Engineer call-out / diagnostic | £70–£120 |
| Replace a faulty thermostat or sensor | £100–£200 |
| Replace a circuit board (PCB) | £250–£500 |
| Replace a heat exchanger or pump | £300–£600 |
| New combi boiler (supply & fit) | £1,800–£3,500 |
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Start with the simple things: power and fuse, the thermostat and programmer settings, and the boiler pressure. A surprising number of "dead" boilers have simply lost power, been switched off at the spur, or aren't being told to fire by the controls. If everything checks out and it still won't run, call a Gas Safe registered engineer.
Yes — pressing the reset button on the front panel is a safe homeowner action. Do it once. If the boiler locks out again, stop resetting it and book an engineer; the lockout is a safety feature and repeatedly clearing it without fixing the cause can be dangerous.
Often, yes. Topping up via the filling loop to around 1.5 bar with the boiler off is a safe homeowner job. But if the pressure keeps dropping, there's a leak in the system that needs a professional to find and repair — topping it up repeatedly only masks the problem.
It usually happens overnight in freezing weather: the boiler locks out, you may hear gurgling, and there's a fault code. If you can safely reach the external pipe, thaw it with warm (not boiling) water and reset the boiler. If it keeps refreezing, an engineer can reroute or insulate the pipe.
A broken boiler is far less stressful when the repair bill is covered. See indicative prices and cover levels across our selected panel of providers.
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