Boiler not firing up: it tries to light, then gives up

What happens when a boiler fires up

Knowing the sequence helps you work out where it is stopping. When the timer or thermostat calls for heat, a modern boiler starts the fan to clear the combustion chamber, checks that air is moving through the flue, then makes a spark and opens the gas valve. An electrode in the burner then has a few seconds to detect a flame. If it does not, the boiler shuts the gas off, tries again, and after a set number of failed attempts locks out and shows a code.

So "not firing up" can mean the boiler never starts that sequence at all, or it starts and fails at the ignition stage. The first is usually power or controls. The second, which this guide is about, is usually gas, air or ignition.

Check 1: is the boiler being asked for heat?

Before assuming an ignition fault, make sure the boiler actually has a reason to fire. Turn the room thermostat up a few degrees above the room temperature, check the programmer is set to on or constant, and run a hot tap if you have a combi. A boiler that is simply not being asked for heat will sit quietly and look broken.

If the display is completely blank, the problem is power rather than ignition. Our guide to a boiler not working covers the fused spur, the consumer unit and what to do after a power cut.

Check 2: the gas supply

Try the hob or any other gas appliance. If nothing lights, the supply is the problem. On a prepayment meter, check the credit and whether emergency credit has run out. At the meter, the emergency control valve handle should run in line with the pipe when it is on. If the gas company or another trade has been working nearby, check the valve was left fully open.

If the hob lights but the flames are small, lazy or lifting, the gas pressure reaching the house may be low. That is not something to adjust yourself, but it is useful to tell the engineer, because a boiler needs more gas than a single hob ring and may be the first appliance to fail.

If you smell gas, do not try to light anything. Open the windows, avoid switches, turn the gas off at the meter if it is safe to do so, leave the property and call the National Gas Emergency Service on 0800 111 999.

Check 3: pressure and the condensate pipe

Most combi and system boilers will not fire if the water pressure is too low. Look at the gauge with the system cool. Around 1 to 1.5 bar is typical, but your user guide gives the right figure for your model. If it is close to zero, top up with the filling loop following the manual, then look for why it dropped. Our guide to a boiler losing pressure explains the usual causes.

In freezing weather, check the plastic condensate pipe where it runs outside. If it has frozen, water backs up into the boiler and it will refuse to fire, often with a gurgling sound. Thaw it with warm, not boiling, water and reset once. Our guide to a frozen condensate pipe shows how to find it and stop it happening again.

Reset once, then stop

Once you have ruled out the gas, the pressure and the condensate pipe, one reset following your boiler's instructions is reasonable. If the cause was an empty meter or a frozen pipe, the boiler should fire normally. Our guide on how to reset a boiler shows where the reset usually is.

If it locks out again, leave it off. Every failed ignition attempt means gas has been released without lighting, and repeated resets on a boiler with an ignition fault make the problem harder to diagnose. Write down the code and what you heard before it stopped. Boiler lockout keeps returning explains why a repeating lockout matters.

Fault codes that mean the boiler failed to light

Most boilers show a specific code when ignition fails. The code tells the engineer where to start, so note it exactly, including any letters and full stops. Some common ones: On Baxi and Potterton boilers the code starts with E1; our E1 boiler fault guide explains how to read the full number.

• Vaillant F.28: ignition failed at start-up. F.29 means it lit, then lost the flame and could not relight.
• Ideal L2: ignition lockout. F2 means the flame was lost after lighting.
• Baxi E133: ignition failure or interrupted gas supply.
• Worcester Bosch EA: no flame detected. A flashing blue light on its own only tells you there is a fault; the code on the display says which.

Why a boiler won't fire up once the basics are fine

With the gas, pressure and condensate ruled out, the cause is inside the boiler or its flue, and this is Gas Safe work only. Do not remove the casing or try to clean parts yourself. An engineer will usually check:

• The ignition and flame-sensing electrode: worn, dirty or wrongly gapped electrodes and cracked leads are among the most common causes.
• The gas valve: whether it opens when told to and passes the right amount of gas.
• The working gas pressure at the boiler, which can be low because of a partly closed valve, an undersized pipe or a faulty meter regulator.
• The fan and air pressure switch or sensor, which must confirm air is moving before ignition is allowed.
• The flue: blocked, damaged or poorly sealed flues stop the boiler proving it is safe to light.
• The condensate trap inside the boiler, and the printed circuit board that controls the sequence.

Getting your boiler firing again

Loyal Plumbers works across London and everywhere inside the M25, 24 hours a day, with 10 vans on the road. Gas work is carried out by Gas Safe registered engineers. When you call, tell us the boiler make and model, the fault code, what you heard before it stopped, the pressure reading and whether your other gas appliances work.

Our call-out charge is £60. The engineer will explain what they have found and what the repair involves before starting. On an older boiler with a string of faults, our guide on whether to repair or replace a boiler helps you weigh the options, but a proper diagnosis comes first. You can read more on our boiler repair and servicing page.