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When a boiler will not fire up, the cause is often something simple: no power, a control not asking for heat, low pressure, or, in a cold snap, a frozen condensate pipe. Work through the safe checks below in order. They take about half an hour and could save you an £80 to £120 call-out. Anything involving the gas supply, the burner or the inside of the boiler needs a Gas Safe registered engineer.
If you smell gas, do not try to light or reset anything. Call the National Gas Emergency Service on 0800 111 999, turn off the gas at the meter if it is safe to reach, open windows and keep away from switches and flames.
What the boiler is telling you
Before touching anything, look at the display and lights. Write down any fault code exactly as shown, then look it up in your manual or on the manufacturer’s website. Many codes point straight at one of the checks below, such as low pressure or a flame failure. The code also saves an engineer time if you do need one.
The safe checks, in order
| Check | What to look for | What to do |
|---|---|---|
| Power | Display blank, no lights | Check the fused spur switch next to the boiler and your consumer unit for a tripped switch |
| Thermostat and timer | Heating not being asked for | Turn the room thermostat up a few degrees above room temperature and check the timer is on |
| Pressure | Gauge below about 1 bar when cold | Top up using the filling loop as your manual describes |
| Gas supply | Other gas appliances dead | Check your hob works and a prepayment meter has credit |
| Condensate pipe | Gurgling noise, frost outside, fault code | Thaw with warm, not boiling, water |
| Reset | Lockout light or code | Reset once, following the manual |
Power
A blank display usually means no power. Check the switch on the fused spur, which is the wall switch next to the boiler, often with a small red light. Then check your consumer unit for a tripped switch. If something trips again as soon as you reset it, leave it off and call an engineer, because that points to an electrical fault.
Thermostat, timer and smart controls
The boiler only fires when something asks for heat. Turn the room thermostat up to a few degrees above the current room temperature. Check the timer or programmer is in an “on” period, and that the clock is right after any power cut. If you have a smart thermostat, check it is online and that its batteries are not flat. On a combi, try running a hot tap: if the boiler fires for hot water but not heating, the boiler is probably fine and the fault is in the controls or a valve.
Pressure
Low pressure is one of the most common reasons a boiler locks out. If the gauge reads below about 1 bar with the system cold, top it up using the filling loop, following your manual. Our guide on a boiler losing pressure covers how to do this and what to do if it keeps dropping.
Gas supply
If your hob or gas fire will not light either, the problem is the supply, not the boiler. On a prepayment meter, check the credit and the emergency credit. If you are in credit and nothing gas works, call your gas supplier. Never touch the gas valve on the boiler or the pipework.
A frozen condensate pipe
Condensing boilers drain acidic water through a plastic pipe, often running outside to a drain. In freezing weather that water can freeze and block the pipe, so the boiler shuts down. Signs include a gurgling noise from the boiler, a fault code, and a hard frost or snow outside.
To thaw it:
- Find where the plastic pipe leaves the house and follow it to the end. The blockage is usually at the end or at a bend.
- Pour warm water slowly along the pipe, or hold a hot water bottle or a cloth soaked in warm water against it. Use warm, never boiling, water, because boiling water can crack the pipe.
- Keep going until you hear or see the water drain.
- Reset the boiler as your manual describes.
If the pipe is high up, do not climb a ladder in icy conditions. Wait or call an engineer.
Reset, once
Most boilers have a reset button or a reset option on the display. Follow your manual. Reset once, and if it locks out again, reset a second time at most. A boiler that keeps locking out is shutting itself down to protect you from a fault. Pressing reset over and over will not fix it.
When it is a job for an engineer
If you have worked through every check and the boiler still will not fire, the cause is probably a part such as the ignition electrode, fan, gas valve, pump or circuit board. These all involve working inside a gas appliance, so they are only for a Gas Safe registered engineer. Book one if:
- The boiler locks out again after one or two resets.
- You hear the boiler try to light but it fails.
- There are scorch marks, soot or a yellow flame.
- Water is leaking from the boiler.
- A switch trips every time the boiler tries to start.
A diagnostic visit is typically £80 to £120, and most repairs that follow are £150 to £450, checked October 2026. See boiler repair cost for typical prices by part. Before anyone starts, check they are Gas Safe registered.
If this is the third breakdown in two years, read repair or replace your boiler before agreeing to a big part. City Boiler connects you with independent local engineers who are Gas Safe registered. See the boiler repair page for more.
Common questions
Why is my boiler not firing up?
The most common causes are no power, a thermostat or timer not calling for heat, low pressure, or a frozen condensate pipe in cold weather. Less often it is a failed part such as the ignition, fan, gas valve or circuit board, which needs a Gas Safe registered engineer.
How do I thaw a frozen condensate pipe?
Pour warm water, not boiling, slowly along the outside section of the plastic pipe, or hold a warm water bottle against it until the ice clears. Then reset the boiler as your manual describes. Boiling water can crack the pipe.
How many times should I reset my boiler?
Once or twice at most, following the instructions in your manual. If it locks out again, the boiler is protecting itself from a fault, and repeated resets will not fix it. Book an engineer.
Could it be the gas supply?
Check that your other gas appliances, such as a hob, work and that a prepayment meter has credit. If nothing gas works and you are in credit, contact your supplier. If you smell gas, call 0800 111 999 at once.