First, the safety line
Before any checklist: if you can smell gas, stop reading. Call the National Gas Emergency line free on 0800 111 999, open the windows, do not operate light switches or anything electrical, and do not smoke. That line is staffed around the clock and making the property safe is their job. Everything below assumes there is no gas smell, just a boiler that will not do its job.
No lights on the display at all
A completely dead display means the boiler is not getting power, and that is an electrics question before it is a boiler question. Check the switched fused spur next to the boiler, the switch that looks like a light switch with a fuse holder, because they get knocked off by accident more often than anyone admits. Then check your consumer unit for a tripped breaker or RCD and reset it once if it has tripped.
This is also the classic aftermath of a power cut. When the electric comes back, some boilers return to life on their own, some sit in a fault state waiting to be reset, and some lose their clock and programmer settings so the heating never gets told to come on. Reset the boiler once using the method in your manual, check the programmer is actually calling for heat at the right time, and give it a few minutes.
If the spur is on, nothing has tripped and the display is still dead, stop there. Repeatedly resetting breakers into a fault is how small problems become big ones, and a dead supply needs proper diagnosis.
Lights on, but no heating and no hot water
Now the boiler has power but will not run, and the display usually tells you why if you let it. Most modern boilers show an error code when they lock out: note it down exactly, because that code can mean the difference between a five-minute fix and a part on order.
Three checks are safe and worth doing. First, pressure: if the gauge reads below about 1 bar the boiler may simply refuse to fire, and topping up to around 1.2 bar through the filling loop is normal homeowner territory, walked through step by step in the pressure guide. Second, the thermostat: flat batteries in a wireless room stat are one of the great unsung causes of "broken" boilers, so swap them and make sure the stat is actually set above the current room temperature. Third, the gas supply: check another gas appliance like a hob. If that is dead too, the problem is the supply, not the boiler, and your gas supplier is the right call.
On a frosty morning, add a fourth: the condensate pipe. If your boiler drains through a plastic pipe down an outside wall, that pipe can freeze overnight and lock the boiler out, often with a gurgling sound. Pouring warm, never boiling, water along the pipe and resetting the boiler frequently brings the whole system back without an engineer.
Heating works but the hot water does not, or the other way round
When exactly half the job works, the fault is usually in the part that switches between the two. On a combi that is typically the diverter valve, the component that swaps the boiler’s output between radiators and taps, and a worn one classically gives you heating but lukewarm water, or hot water only while the heating happens to be running. On a system with a cylinder, a stuck motorised valve produces the same split symptoms.
There is no safe DIY fix for either. They are engineer jobs, but usefully specific ones: tell Lloyd which half works and which does not, plus any error code, and he arrives knowing what he is probably looking at, often with the likely part in the van.
When to stop and call a Gas Safe engineer
Call when the checks above have not moved it: an error code that comes back after one reset, lockouts that repeat over days, water where it should not be, banging or kettling noises, or pressure that keeps falling after top-ups. Do not keep resetting a boiler into the same fault, and do not open the case, ever. Anything behind the case is legally Gas Safe registered work.
If you are in Bridgend county or the surrounding patch, call 07712 646488 with the error code and the symptoms. Lloyd answers his own phone during working hours, tells you honestly whether it sounds like a repair or something bigger, and prices the visit before he sets off. The repair-or-replace question gets the same honesty: most faults on a youngish boiler are worth fixing, and when they are not, a fixed replacement price from £1,900 is on the table in about 90 seconds so you can decide with both numbers in front of you.
