Boiler Not Turning On? What to Check
When a boiler will not turn on, the cause is often something you can check in a few minutes: power, the controls, the water pressure, the gas supply or a frozen condensate pipe. Work through the list below in order. If the boiler still will not fire, or it lights and then locks out, it needs a Gas Safe engineer.
Check in this order
| Check | What to look for |
|---|---|
| 1. Power | Display lit? Check the fused spur switch near the boiler and your fuse box |
| 2. Controls | Programmer on, times right after a power cut or clock change, thermostat turned up above room temperature |
| 3. Pressure | Gauge in the normal range (about 1 to 1.5 bar cold on most boilers) |
| 4. Gas | Does the hob light? Prepayment credit? Meter valve in line with the pipe? |
| 5. Condensate | In frosty weather, is the outside pipe frozen? |
| 6. Fault code | Note the code, check your manual, reset once |
Power and controls
A blank display usually means no power: check the switched spur next to the boiler and the circuit breaker or fuse in your consumer unit. If the display is lit but nothing happens, the boiler is probably not being asked for heat. Check the programmer or smart control is on and its clock is right, especially after a power cut, and turn the room thermostat up above the current room temperature. On a system with a cylinder, check the hot water and heating channels separately.
Pressure
Most combi and system boilers will not run if the pressure is too low. If the gauge is below the range in your manual, top up using the filling loop following the manual, then reset. If the pressure keeps falling, see our guide to boiler pressure that keeps dropping.
Gas supply and frozen condensate
If the hob will not light either, it is the gas supply, not the boiler: top up a prepayment meter, check the meter control valve, and if neighbours have lost gas too, wait for the network to restore it. In cold weather a frozen condensate pipe is the most common reason a boiler that was fine yesterday will not fire; thaw the outside section with warm water from the ground, then reset. Our guides to L2 and F28 cover these lockouts in detail.
Wireless thermostats and smart controls
A common cause of a boiler that will not come on is a wireless room thermostat with flat batteries, or a receiver that has lost its pairing with the thermostat. Change the batteries first and check the receiver, usually a small box near the boiler, shows a link. With a smart control, check the app shows the system online and that a schedule or holiday mode is not holding the heating off. If the boiler fires when you use its own controls or a manual override, the fault is in the controls, not the boiler.
When to stop and call an engineer
Reset once. If the boiler locks out again, shows the same fault code, or you can hear it trying to light without success, stop and book a boiler repair. Anything inside the case is gas work for a Gas Safe registered engineer. And if you smell gas at any point, open windows, avoid switches, leave the property and call the National Gas Emergency Service on 0800 111 999.
Common questions
Why won't my boiler turn on but the display is lit?
Usually the controls are not calling for heat: check the programmer times, the room thermostat setting and, on a cylinder system, the hot water channel. Low pressure or a fault code can also stop it firing.
Why won't my boiler fire up in cold weather?
The most common cause is a frozen condensate pipe outside. Thaw it with warm, not boiling, water from the ground, then reset the boiler.
How do I reset my boiler?
Follow the reset procedure in your boiler's manual; most have a reset button or a reset position on a dial. Reset once, and if the fault returns, call an engineer rather than resetting repeatedly.
Should I call a plumber or a gas engineer?
For anything inside the boiler, a Gas Safe registered engineer: working on gas appliances for payment legally requires Gas Safe registration.
Updated 15 October 2026
