Before you ring anyone, there are six things worth checking. They take about ten minutes between them and they're the same six I'd ask you about on a call.
Quite often one of them is the answer. When it isn't, you'll at least know what to tell whoever turns up, which stops you paying someone to find out what you could have told them.
They're in order for a reason. Work down.
If you can smell gas, or your CO alarm is going off, stop reading. Leave the property and ring the National Gas Emergency Service on 0800 111 999. In Northern Ireland it's 0800 002 001.
If there's a burning smell, sparking, or scorch marks indoors, turn the circuit off at the consumer unit if it's safe to do so and get an electrician. If there's fire or anyone's in danger, ring 999.
If you've a power cut, or a damaged cable or substation outside, ring 105. It's free, it's 24 hours, and it puts you straight through to your local network operator. Most people don't know it exists.
Go and stand in front of the boiler.
Is there any light on it at all? If the display is dead, check the switch on the wall beside it, the fused spur, and whether a breaker has tripped in the consumer unit. Plenty of "broken boilers" are a switch somebody knocked.
If it is showing something, write down exactly what it says. Every character, including any letters. That code is the most useful thing you can tell anyone.
Then press reset. Once. If it fires and stays running, good. If it locks straight back out, stop pressing it. Repeated resetting doesn't fix anything and it makes the fault harder to trace.
Be specific, because "it's not working" covers about thirty different faults.
Run a hot tap to test the hot water, not the shower.
This one catches people out constantly. An electric shower heats its own water and has nothing whatsoever to do with your boiler. A mixer shower has its own cartridge that can fail on its own.
I've been called out to plenty of "no hot water" jobs where the boiler was fine and the shower was the problem. Run the kitchen tap. If it does turn out to be the shower, I've written that one up separately.
And if the radiators are heating but some of them stay cold, or only part of one gets hot, that's cold radiators rather than a boiler fault.
Sounds obvious. It isn't.
If you're on a prepayment meter, is there credit on it? Has an emergency credit run out overnight?
Either way, go and light a gas hob ring. If the hob lights normally you've got gas at the property. If it doesn't, the boiler was never the problem.
A boiler in perfect working order does nothing at all if nothing is telling it to.
Find the gauge on the boiler, or sometimes on the pipework below it.
Most want to sit between 1 and 1.5 bar when the system is cold. Under about 0.8 and the boiler will usually refuse to fire.
Note the number down either way, because "it's fine" and "it's at 0.4" lead to two completely different conversations. And if it's reading high rather than low, over 2.5 bar once the heating's been on, that's the opposite problem and it's covered on the boiler pressure too high page.
This is the question almost nobody asks themselves, and it solves more faults than the other five put together.
Whatever the answer is, tell whoever you speak to. It's usually the fastest route to the fault.
Once you've been through those, you'll usually be in one of these:
Pressure is low, or it drops again after topping up. That's its own thing, and there's a page on it here.
Cold weather, boiler fires then cuts out, maybe a gurgle. That's almost always the condensate, and there's a page on that too.
No light, no display, nothing. Fused spur, its fuse, or a tripped breaker. If none of those, stop there.
Everything looks right and it still won't fire. Now it's worth someone looking, because you've ruled out everything you safely can.
Get someone if:
Past that point you're into things that need testing rather than looking at, and guessing gets expensive. The most common way people waste money on a boiler is replacing the part the code names, when the code is reporting something upstream of it.
I'm Carlo Cennerazzo, twenty-three years in the trade. Point your phone at the boiler and I'll go through it with you, tell you what's wrong, whether you can sort it yourself, and what it should cost if you can't.
£20, no callout charge, same day when there's a slot free. A written report follows afterwards, so you've got it in black and white rather than trying to remember.
And if you've already had someone out and you're holding a quote you're not sure about, that's worth a call too. I've no interest in who does the work, so I'll tell you straight whether the price is fair and whether it's even the right job.