First, is it really a power fault?
Before assuming the worst, rule out the simple explanations. Some boilers dim or switch off the display after a period with no activity to save energy, and the screen wakes up when you press a button. On others the display shows very little in standby. Check the manual for your model to see what a normal standby screen looks like.
Then check the power more widely:
- Are other lights and sockets in the house working? If nothing works, it is a power cut or a problem with your main supply, not the boiler.
- If you have a separate timer, programmer or wireless thermostat receiver, is it showing anything? In many homes the controls share the boiler supply, so if they are dead too, power to the heating has gone.
- Does a smart thermostat hub near the boiler have no lights? That is another sign the supply is missing.
Check the fused spur
Most boilers are wired into a fused connection unit, usually called a fused spur. It is a square switch plate on the wall near the boiler, often with a red neon light and a small fuse holder on the front. Some are labelled "Boiler" or "Central heating". On older installations it may be in an airing cupboard or next to the hot water cylinder.
- Check the switch is on. Spurs sometimes get knocked off by accident, or switched off during decorating and forgotten.
- If the switch is on but the neon light is off, the fuse may have blown or the circuit may be off at the consumer unit.
- To check the fuse, switch the spur off first. Pull out the fuse carrier (a small flat-head screwdriver helps ease it out), remove the cartridge fuse and fit a new one of the same rating. Boilers are commonly on a 3 amp fuse, but always match the rating printed on the old fuse or in the installation manual.
- Refit the carrier, switch the spur back on and see whether the display returns.
A few boilers are connected with a normal plug instead. In that case the fuse is in the plug top: switch off and unplug it, change the fuse for one of the same rating and plug it back in.
Only check the fuse in the spur or plug top. Never open the boiler casing to look at the internal fuses, wiring or circuit board. If a new fuse blows straight away, do not keep replacing it. That means a fault is drawing too much current, and it needs an engineer.
Check the consumer unit
The consumer unit (fuse box) is usually near the front door, under the stairs or in a garage. Look for a switch that has dropped to the off position. The breaker for the boiler might be labelled "Boiler" or "Heating", or the boiler may share a general sockets or lighting circuit.
- If a single circuit breaker (MCB) has tripped, switch it back on once.
- If a larger RCD switch has tripped and taken out several circuits, switch it back on once.
- If it trips again, leave it off. Switch the boiler spur off, then try the breaker again. If it now stays on, the fault is probably in the boiler or its wiring and needs a Gas Safe registered engineer. If it still trips with the boiler isolated, call an electrician.
Water can cause trips too. A leak dripping onto wiring or into the spur, or condensation in a cold loft, can make an RCD trip. If you see water near the electrics, keep the power off and get it checked. Our guide to a boiler leaking water explains what to look for.
When the fault is inside the boiler
If the spur light is on, the breaker is on and the boiler is still dead, power is reaching the boiler but not getting through. Typical internal causes include:
- a blown fuse on the main circuit board, often after a power surge, a thunderstorm or a fault in another part such as the pump or fan
- a failed printed circuit board (PCB) or its power supply section
- a faulty display board or its connecting cable, where the boiler may still run but the screen stays blank
- loose or damaged wiring between the spur and the boiler
- water damage to the electrics from an internal leak
All of these are jobs for a Gas Safe registered engineer. The engineer will check the incoming supply, test the internal fuses and components and find out why a fuse blew before replacing it, because a new fuse on its own often blows again. If the PCB is failing but still partly working, you may see board faults such as Ideal F9 or Vaillant F63 instead of a blank screen.
Quick checklist
| Check | What you are looking for | Next step |
|---|---|---|
| Power elsewhere in the house | Other lights and sockets working | If nothing works, report a power cut by calling 105 |
| Display standby mode | Screen wakes when you press a button | Nothing to fix |
| Fused spur switch and neon | Switch on, light lit | Switch on, or check the fuse |
| Fuse in the spur or plug top | Fuse intact | Replace with the same rating, once |
| Consumer unit | No tripped MCB or RCD | Reset once, electrician if it trips again |
| Thermostat and timer | Calling for heat, batteries not flat | Replace batteries or adjust settings |
Typical repair costs in the UK
| Job | Typical UK cost |
|---|---|
| Replacement cartridge fuse for a spur or plug | A few pounds |
| Engineer call-out and diagnosis | Typically £60 to £120 |
| Internal fuse replaced and cause found | Typically £80 to £150 |
| Display board replaced | Typically £150 to £350 |
| Main PCB replaced | Typically £250 to £600 |
| Electrician to trace a tripping circuit | Typically £60 to £150 |
PCB costs vary the most. A board for a common Ideal or Baxi combi sits towards the lower end, while boards for premium brands or older discontinued models cost more, and some boards must be configured to the exact boiler model on site. London labour rates, out-of-hours visits and difficult access also push prices up. If the boiler is more than about 10 to 15 years old and needs a new PCB, it is worth getting a replacement boiler quote as well. Keeping the boiler serviced, as explained in our boiler service cost guide, lets an engineer spot scorched connections and small leaks before they take out the electrics.