Boiler Not Igniting: Causes & Fixes

Before anything else: if you can smell gas, do not attempt any troubleshooting. Do not operate any electrical switches, plugs, or open flames. If it is safe to do so, turn off the gas supply at the meter, open windows, and call the National Gas Emergency Line immediately on 0800 111 999.

A boiler that refuses to ignite leaves you with no heating or hot water, and it tends to happen at the worst possible moments. The good news is that the majority of ignition failures come down to a handful of well-understood causes, several of which have straightforward DIY fixes that cost nothing.

Knowing how to identify a combi boiler versus a system or regular boiler is the first useful step, because the fault codes, reset procedures, and common ignition issues differ between boiler types. If your display is completely dead rather than showing a fault, the no power issues with boilers guide covers that specific scenario in detail. And if your controls or timer seem unresponsive, boiler timer troubleshooting methods can help rule out a programming or scheduling issue before you look at the boiler itself.

This guide covers every major reason a boiler fails to ignite, explains what is happening inside the heating system, and gives clear step-by-step actions for what you can safely attempt yourself, and where the line is for calling a Gas Safe engineer.

Quick Diagnosis: What Does Your Boiler Look Like Right Now?

Use the table below to match what you are seeing or hearing to the most likely cause before reading the detail sections.

What you see / hear	Most likely cause	DIY or engineer?
Completely dead display; nothing on screen	No power: fuse spur tripped, blown fuse, or failed PCB	DIY: check fuse spur and fuse box. Engineer: if PCB
Display active but boiler attempts to light and fails	Gas supply issue, faulty electrode, dirty flame sensor	DIY: check gas supply. Engineer: electrode/sensor work
Fault code showing on display	Depends on code: pressure, ignition, condensate, fan	Look up code in manual; most need engineer
Boiler fires briefly then cuts out	Flame detected then lost: dirty flame sensor, gas valve, or PCB	Engineer only
No hot water or heating; cold snap overnight	Frozen condensate pipe blocking drainage	DIY: thaw with warm water; reset boiler
Thermostat shows demand but boiler silent	Dead thermostat batteries or receiver not communicating	DIY: replace batteries in both units
5+ clicks when boiler tries to light	Ignition electrode or lead fault	Engineer: electrode inspection and replacement

One reset attempt is the rule. If the fault code returns immediately after resetting, stop and call a Gas Safe engineer. Repeated resets without resolving the underlying issue can cause damage to components and create unsafe conditions.

View Our Quick Video Guide Here:

1. Low or No Gas Supply to the Boiler

A lack of gas is the first thing to rule out because it is simple to check and surprisingly common. Gas supply interruptions can affect just the boiler, just your property, or the wider area depending on the cause.

What Causes a Gas Supply Problem?

• The emergency control valve (ECV) at the gas meter has been knocked or accidentally turned off
• A problem with the gas supply pipework into the property
• A temporary interruption from the National Grid (more likely during cold snaps when demand is high)
• A frozen gas meter regulator in extreme winter conditions, which can restrict gas flow
• A prepayment meter that has run out of credit

How to Check and Fix a Gas Supply Issue

1. Turn on a gas hob or cooker. If it also has no gas, the whole house supply is affected and the issue is not with the boiler itself.
2. Go to your gas meter and check that the lever on the emergency control valve is aligned with the pipe (parallel = open; perpendicular = closed). If it has been turned off, turn it back to the open position.
3. If the meter is outside in freezing conditions and the pipe looks frosted or iced, gently pour warm water over the regulator and pipes. Never use boiling water.
4. If the supply appears fine but the boiler still has no gas, call your gas supplier to check for a local supply interruption. If you smell gas at any point, call the National Gas Emergency Line on 0800 111 999.

2. Frozen Condensate Pipe

This is one of the most frequent causes of boiler ignition failure during UK winters, and it is one you can fix yourself in most cases. Almost all modern condensing boilers have an external white plastic condensate pipe that carries acidic wastewater from the boiler to an outside drain.

When temperatures drop below zero for several hours, the water inside the external section of this pipe freezes and creates a blockage. The boiler detects this via a pressure switch and locks out to protect itself. It will not attempt to ignite while the blockage is present.

How to Fix a Frozen Condensate Pipe

1. Find the condensate pipe: it is typically a white plastic pipe (22 mm or 32 mm diameter) leaving the boiler through an external wall and running down to a drain or soil pipe.
2. Identify the frozen section. It is usually the bend or lowest section of the external run, where water pools and freezes fastest.
3. Pour warm water (not boiling: boiling water can crack plastic pipe) along the frozen section until you hear water running or gurgling inside the pipe.
4. Once thawed, reset the boiler via the reset button on its control panel and allow it to complete its start-up sequence.
5. After the boiler is running again, fit foam pipe insulation lagging over the external section to reduce the risk of refreezing. For pipes that freeze every winter, trace heating cable is a more permanent solution.

3. Faulty Ignition Leads or Electrodes

The spark generator and ignition electrodes are what create the spark that lights the gas at the burner. If either component has degraded, the boiler will attempt ignition (you will hear clicking) but fail to light. Most boilers make three to four attempts before entering lockout. More than five or six clicks per attempt usually points to an electrode or lead fault.

What Causes Ignition Electrode Problems?

• Carbon or soot build-up on the electrode tip, which reduces spark quality
• A cracked ceramic insulator around the electrode body
• Loose, corroded, or damaged ignition leads between the spark generator and electrode
• Electrode gap too wide or too narrow: the correct gap is typically 3 to 4 mm

What You Can Safely Check

Gas Safe regulations allow homeowners to remove the boiler casing on most models to inspect accessible components. If you are confident doing so, you can visually inspect the electrode and leads:

• Check that the ignition leads are firmly pushed onto their terminals at both ends. Vibration over time can loosen them.
• Inspect the electrode tip for soot or carbon deposit. A gentle clean with fine sandpaper can restore conductivity if the tip is dirty.
• Check the ceramic insulator for cracks. If it is cracked, the electrode must be replaced by a Gas Safe engineer.
• If the gap looks obviously wrong, do not attempt to adjust it yourself without the manufacturer’s specification: an incorrect gap is worse than a slightly worn one.

 

If cleaning or reseating the leads does not resolve the fault, the electrode or lead set needs replacing. This is engineer work.

4. Blocked or Dirty Flame Detection Probe

The flame detection probe (also called the ionisation probe) sits in the burner flame and detects whether combustion has successfully started. It does this by measuring a tiny electrical current through the flame. If the probe tip is coated with soot or corrosion, it cannot detect the flame accurately and the boiler shuts the gas valve as a safety measure, even when ignition has occurred normally.

The symptom is a boiler that fires briefly and then cuts out after a second or two, rather than one that clicks without ever lighting.

What Causes Flame Detection Problems?

• Soot or oxidisation on the metal probe tip
• Probe that has shifted slightly out of position relative to the flame
• Faulty PCB not reading the probe signal correctly

What You Can Do

If you have removed the boiler casing and can access the probe tip, clean the metal surface gently with fine emery cloth to remove any oxidisation or carbon deposit. This is often enough to restore the signal. Do not adjust the probe’s physical position unless you have the exact manufacturer specification for its alignment: misalignment causes the same symptom as contamination.

If cleaning does not clear the fault, the probe or PCB needs professional diagnosis.

5. Gas Valve or PCB Failure

The gas valve opens to admit gas to the burner when the PCB signals it to do so. The PCB (printed circuit board) is the boiler’s control brain: it reads all the safety sensor signals and decides whether it is safe to proceed with ignition. If either component fails, ignition cannot occur regardless of the state of the electrodes or flame sensors.

What Causes Gas Valve or PCB Issues?

• Age and wear: both components have a typical lifespan of 10 to 12 years in a well-maintained boiler
• Power surges or electrical spikes, particularly in areas with unstable mains voltage
• Water ingress from an internal boiler leak reaching the PCB or valve connections
• A failed modulator on the gas valve, causing it to stay closed or open at the wrong pressure

 

Gas valve and PCB faults are not DIY repairs. A Gas Safe engineer with appropriate diagnostic tools is required to test these components accurately and replace them safely. Attempting to work on a gas valve yourself is illegal under UK gas safety regulations.

If your boiler is over 10 to 12 years old and needs a PCB or gas valve, it is worth getting a replacement quote alongside the repair estimate, as the cost of major components can approach the value of an older boiler.

6. Air Pressure Switch (APS) Fault

The air pressure switch confirms that the fan is running and creating the correct draught through the flue before allowing the gas valve to open. It is a safety device that prevents the boiler from attempting ignition if there is any risk that combustion gases could be released into the property.

What Causes APS Problems?

• Blocked flue or air intake terminal (leaves, birds’ nests, debris, or snow at the external grille)
• Fan failure: the APS will not trip if the fan is not generating sufficient airflow
• Split, kinked, or blocked rubber tubing connecting the APS to the flue/fan assembly
• The APS switch itself failing mechanically or electrically

What You Can Check

Visually inspect the external flue terminal from ground level. If accessible and safe, clear any visible debris, leaves, or ice from the grille. This is the only APS-related check that is safe to perform without Gas Safe registration.

If the flue terminal is clear but the APS fault persists, this is engineer-only territory. Checking the internal tubing, fan speed, and switch operation requires opening the boiler casing and using diagnostic tools.

7. Incorrect Gas or Burner Pressure Settings

If the gas pressure at the burner is outside the manufacturer’s specified range, the boiler will either fail to light or light and then lock out immediately. This is a less common cause of ignition failure but worth mentioning because it can follow a recent boiler service or installation if commissioning was not carried out correctly.

What Causes Incorrect Burner Pressure?

• Recent service or installation where burner pressure was not correctly set and verified with a flue gas analyser
• A gas valve modulator that has drifted from its correct position over time
• Incorrect gas supply pressure from the meter, which can affect burner performance on high-output boilers

 

Adjusting burner or gas supply pressure requires a Gas Safe engineer with a combustion analyser. It is not a homeowner task under any circumstances.

8. Power Supply and Control Issues

A significant number of boiler ignition failures are not ignition faults at all: they are power or control issues that prevent the boiler from ever receiving the signal to start. These are worth checking first because they take seconds to diagnose and cost nothing to fix.

• Fuse spur: the isolating switch next to the boiler (or in the fuse box) may have tripped. Check and reset it.
• Consumer unit: check that the boiler’s circuit breaker or fuse has not tripped. Reset if so; if it trips again immediately, call an electrician.
• Thermostat batteries: a wireless thermostat with dead batteries sends no demand signal to the boiler. Replace the batteries in both the thermostat and the receiver unit (often a separate box near the boiler).
• Programmer schedule: check that the programmer has not lost its settings after a power cut. Many digital programmers revert to standby after a power interruption. Reprogramme the schedule if the clock has reset to 00:00.
• Smart thermostat: some smart thermostats have an away or holiday mode that suppresses heating. Check the app or display to confirm the thermostat is actually calling for heat.

 

The no power issues with boilers guide covers the power supply checks in more detail if the boiler display is completely dark.

How to Reset Your Boiler After Fixing the Issue

Most modern boilers have a reset button, typically marked with a flame symbol or a circular arrow. Press and hold it for 3 to 10 seconds (check your specific model manual for the exact procedure, as it differs between Worcester Bosch, Vaillant, Ideal, Baxi, and other brands).

The boiler will go through a start-up sequence: you should hear the fan start, then a click as the spark generator fires, and then the burner ignite. The whole sequence typically takes 30 to 60 seconds.

Attempt one reset only after any fix. If the fault code returns or the boiler fails to start again, stop resetting and call a Gas Safe engineer. Persistent lockouts indicate an underlying fault that repeated resets will not resolve and may make worse.

Preventing Boiler Ignition Problems in the Future

• Annual service by a Gas Safe engineer. This is the single most effective prevention step: a full service catches electrode wear, soot build-up, flue integrity issues, and pressure problems before they cause a lockout. Annual servicing is also a condition of most manufacturer warranties.
• Insulate the condensate pipe. Fit foam lagging on the external section before winter. For repeat freeze-ups, trace heating cable provides more reliable protection.
• Fit a magnetic system filter (such as a Magnaclean or Fernox TF1) to keep sludge and magnetite away from internal components including electrodes and the flame sensor.
• Keep the flue terminal clear. Check it before each heating season for debris, leaves, or bird activity.
• Fit a surge protector on the boiler’s electrical supply if you are in an area with unstable mains voltage. Power surges are a common cause of PCB damage.
• Replace thermostat batteries annually. Do it at the same time you book the boiler service to make it a single routine.

When You Must Call a Gas Safe Engineer

Some of the checks above are safe for any homeowner. The following situations are strictly Gas Safe registered engineer territory:

• You can smell gas at any point. Turn off the supply at the meter, open windows, leave the property, and call 0800 111 999.
• The boiler is leaking water. Water near electrical and gas components creates multiple hazards. For background on what boiler leaks indicate, the common boiler leak causes guide covers the main reasons this happens.
• You see black soot marks around the boiler or flue terminal. This indicates incomplete combustion and potential carbon monoxide risk.
• The flame is yellow or orange rather than blue when visible. A yellow or orange flame indicates incomplete combustion.
• The fault returns after a single reset attempt.
• You are in any doubt at all. Gas appliances are not the place for trial and error.

 

Working on gas appliances without Gas Safe registration is illegal in the UK and can invalidate your home insurance and any remaining manufacturer warranty. You can verify an engineer’s Gas Safe registration at gassaferegister.co.uk.

If your boiler is repeatedly requiring callouts or is approaching the end of its expected lifespan, it is also worth considering boiler relocation cost considerations if a new installation would benefit from a different location in the property.

FAQs

Will my home insurance cover a boiler that won’t ignite?

Most standard buildings and contents policies do not cover boiler breakdown. Cover for sudden mechanical failure typically requires a dedicated boiler cover plan or a home emergency add-on. Check your policy wording for the phrase ‘sudden and unforeseen mechanical breakdown’: this is the standard exclusion language and its presence means you are not covered without the add-on. If you are thinking about what cover you have or need, it is worth reviewing your options before the next heating season rather than after a breakdown.

How long can I safely leave my boiler locked out before calling someone?

In winter, and particularly if there are vulnerable people, elderly residents, or young children in the property, treat any heating failure as urgent and seek same-day attention. In milder months, 24 to 48 hours is a reasonable window for a non-emergency appointment. Do not leave the boiler locked out indefinitely and keep resetting it: if the underlying fault involves a component operating outside safe parameters, continued attempts to start it can cause secondary damage.

My boiler shows no fault code at all — just completely dead. Is this still an ignition problem?

Not necessarily. A completely dark display usually indicates a power supply issue rather than an ignition fault: the most common causes are the fuse spur being tripped, a blown internal boiler fuse, or a PCB failure. Start by checking the fuse spur beside the boiler and the relevant circuit breaker in your consumer unit. If power is confirmed and the display is still dead, the PCB has likely failed and needs professional diagnosis. A frozen condensate pipe can occasionally produce a dead-looking display if it has caused a pressure switch to lock out before the display initialised.

Is it normal to hear 5-6 clicking sounds when the boiler tries to light?

Three to four clicks per ignition attempt is normal while the spark generator fires to light the burner. More than five or six clicks per attempt usually indicates the spark is not reaching the burner effectively: the most common causes are a dirty electrode tip, a cracked ceramic insulator, or a loose ignition lead. If you consistently hear six or more clicks across multiple attempts, the ignition components need inspection.

Can a wireless thermostat battery cause the boiler not to fire up?

Yes, and this is one of the most overlooked causes of boiler ignition failure. If the room thermostat or its separate receiver unit has flat batteries, the boiler never receives a heat demand signal and will not attempt ignition regardless of the boiler’s own condition. Replace the batteries in both the thermostat handset and the receiver (usually a separate white box mounted near or on the boiler) before assuming there is a fault with the boiler itself.

What does it mean if my boiler ignites but then cuts out after a few seconds?

This pattern, where the boiler lights briefly and then shuts down, usually points to a flame detection failure rather than an ignition failure. The most common cause is a dirty or oxidised flame detection probe (ionisation probe) that cannot read the flame signal reliably. Clean the probe tip with fine emery cloth. If the problem persists, the probe, the gas valve, or the PCB may be at fault and a Gas Safe engineer should diagnose it.