First: is it safe?
Before you touch anything, rule out an emergency. If you can smell gas, your carbon monoxide alarm is sounding, water is pouring from inside the boiler casing or there is a burning smell, stop here. These are not problems to troubleshoot.
If you smell gas, do not switch lights or appliances on or off. Open doors and windows, turn off the gas at the meter if you can, leave the property and call the National Gas Emergency Service on 0800 111 999 (the number given on GOV.UK). If a carbon monoxide alarm sounds, turn the boiler off, get everyone into fresh air and call the same number. Our guide to a smell of gas near the boiler explains what happens next.
If none of those apply, work through the checks below. They are in the order that engineers usually think about a boiler that has stopped: power, then gas, then water, then the signal telling the boiler to run, then the boiler's own report of what went wrong.
What to check if your boiler is not working
Start by noting exactly what you see, because it narrows the cause straight away. A blank display, a flashing code, a boiler that runs but gives no heat and a boiler that works for hot water only are four different problems.
| What you see | Most likely area | Start with |
|---|---|---|
| No lights, blank display | Power supply | Check 1: power |
| Other gas appliances dead too | Gas supply or meter | Check 2: gas supply |
| Gauge below 1 bar | Low water pressure | Check 3: pressure |
| Display on, no code, boiler silent | Thermostat, timer or mode | Check 4: controls |
| Gurgling, freezing weather, lockout | Frozen condensate pipe | Check 5: condensate |
| Fault code or flashing light | Lockout or blocking fault | Check 6: fault code |
| Hot water works, radiators cold | Heating controls or valves | Heating section below |
| Radiators work, no hot water | Diverter valve or hot water side | No hot water guide in related guides |
1. Power
A boiler needs electricity for its fan, pump and controls, even though it burns gas. Check the fused spur switch next to the boiler is on, check the plug if it has one, and look at the consumer unit for a tripped switch. If there was a power cut, the boiler may need one reset, and the timer may have lost its clock. A display that stays blank with power on points to a blown fuse or an internal fault, which our guide to a boiler with no power covers.
2. Gas supply
Try the gas hob or another gas appliance. If that does not light either, the problem is the supply, not the boiler. On a prepayment meter, check you have credit; British Gas advises that when a meter runs out and switches off, the boiler may need resetting afterwards using the manufacturer's instructions. At the meter, the emergency control valve handle should be in line with the pipe. Some newer boilers also have a gas isolation valve underneath, which should be open.
3. Water pressure
Most sealed systems need around 1 to 1.5 bar on the gauge with the heating off and cold, and manufacturer manuals from Worcester Bosch, Vaillant, Ideal and Baxi all use figures in that region. If it falls well below that, many boilers refuse to fire and show a low pressure code. If yours is low, our guide to boiler pressure too low shows how to top it up with the filling loop.
4. Thermostat, timer and mode
If the boiler has power and pressure but does nothing, it may simply not be asked to run. Turn the room thermostat a few degrees above room temperature, make sure the timer or programmer is in an on period or on boost, and check the boiler is not in summer mode, which switches off the heating. Replace the batteries in a wireless thermostat. If it still will not respond, see our guide to a boiler not responding to the thermostat.
5. Condensate pipe
Every modern condensing boiler drains acidic water through a plastic pipe. In freezing weather an outside section can ice up, the water backs up and the boiler locks out, often after a gurgling noise. Thawing it with warm, not boiling, water is a safe DIY job, set out in our frozen condensate pipe guide.
6. Fault code
Write the code down before you reset anything. The code tells you, or the engineer, what the boiler detected. Look it up in our boiler fault codes library, which covers Vaillant, Worcester Bosch, Ideal, Baxi, Glow-worm, Vokera, Viessmann and Potterton.
7. One reset
Once you have dealt with any obvious cause, reset the boiler as the user manual describes. Stop after 2 or 3 attempts: if it locks out again, the cause is still there, and repeated resets only hide it.
Boiler not turning on at all
A boiler that will not turn on at all, with no lights, no noise and no display, almost always has a power problem rather than a gas or water one. The checks are the fused spur, the plug, the consumer unit and any external controls that share the same supply. If a smart thermostat receiver near the boiler is also dead, the power to both has gone.
A boiler that turns on, shows its normal display and then does nothing is a different case. The power is fine, so move on to the thermostat, timer and pressure checks above. If the display shows it is trying to light, see our guide to a boiler not firing up.
Take a photo of the display and the data badge (usually on the underside or inside the front flap) before you call anyone. The make, model and code let an engineer bring the likely part on the first visit.
Heating not working at all
If the heating is not working and the hot water is fine, the boiler itself is healthy and the fault is in what calls for heat or where the heat goes: the thermostat, the timer, summer mode, a diverter valve on a combi or a zone valve on a system boiler. That exact case has its own guide: heating not working but hot water is.
If both heating and hot water have stopped, work through the seven checks, because something is stopping the boiler as a whole. If the radiators get warm but the house stays cold, the boiler is working and the question is flow temperature, balancing or radiator size rather than a breakdown.
Why has my boiler stopped working?
A boiler usually stops because it has detected something outside its safe limits and shut itself down. That is the design working as intended. The common triggers are:
- water pressure dropping too low, often after bleeding radiators or because of a small leak
- a frozen or blocked condensate pipe in cold weather
- no gas reaching the burner, from a meter problem, prepayment credit running out or a closed valve
- a failed ignition, where the boiler tries to light several times and then locks out
- overheating, often from poor circulation caused by sludge, air or a failing pump
- loss of power, including a tripped circuit or a blown fuse in the spur
- a failed component such as a fan, sensor, pump or printed circuit board (PCB)
The first four you can often check or fix yourself. The last three need an engineer, and the fault code is the best clue to which one it is.
Should I turn my boiler off if it is not working?
You do not need to turn a boiler off just because it has stopped, unless you see a warning sign. Switch it off at the controls and the fused spur if there is a gas smell, a carbon monoxide alarm, water coming from inside the casing, scorch marks, a burning smell or repeated banging.
Otherwise, leaving it powered is often better, especially in winter. Built-in frost protection needs the boiler switched on: the Vaillant ecoTEC instructions for use, for example, say the function is only active when the main switch is on and the boiler is connected to the electrical supply, and that it protects the boiler itself rather than the whole system. Other brands' manuals say much the same. Keeping it powered also keeps the fault on the display for the engineer to see.
When to call an engineer
Call a Gas Safe registered engineer if the boiler still will not run after these checks, if it locks out again after 2 or 3 resets, or if the code points to a part inside the casing. Check the engineer's Gas Safe ID card, and make sure it lists the work they are doing. For an oil boiler, call an OFTEC-registered technician instead. For wiring faults outside the boiler, such as a spur that keeps tripping, a qualified electrician can help.
If you rent, report the fault to your landlord or letting agent straight away, in writing. Tell whoever attends which checks you have already done and what the display shows. Our related guide to boiler repair costs lists typical UK figures for common parts, which helps you judge a quote, and if the boiler is old and breaking down often, ask the engineer whether a large repair is still worth it.