Boiler Pilot Light Keeps Going Out: Causes and Fixes for London Homes
Most modern London boilers have electronic ignition — no pilot light. But if yours has one and it keeps going out, here are the causes and whether to call a Gas Safe engineer.
The most likely cause is a faulty thermocouple (costs £50–£100 to replace). Other causes include draughts, a dirty pilot jet, or a failing gas valve. Never attempt gas component repairs yourself — call a Gas Safe registered engineer.
Key Takeaways
- A faulty thermocouple is the most common reason a boiler pilot light keeps going out — it's a straightforward Gas Safe repair costing £50–£100.
- Most boilers installed in London after 2004 use electronic ignition and have no pilot light at all — check your boiler model before diagnosing.
- Never attempt to repair gas components yourself; it is illegal and dangerous without Gas Safe registration.
- Draughts from poorly sealed Victorian terraced houses are a surprisingly common cause in older London properties.
Does Your Boiler Actually Have a Pilot Light?
Before diagnosing why your boiler pilot light keeps going out, it is worth checking whether your boiler has one at all. The vast majority of boilers installed in London since the mid-2000s use electronic ignition — there is no standing pilot flame. Instead, a spark ignites the burner on demand when you turn on heating or hot water. If your boiler is clicking or failing to fire up, that is an ignition fault rather than a pilot light problem.
Older boilers — particularly those still running in Victorian terraced houses across Hackney, Islington, Lewisham, and similar inner-London boroughs — may well have a traditional pilot light. These are typically floor-standing or back boilers hidden behind a gas fire, and they have been out of production for many years. If your boiler was installed before around 2004 and you can see a small flame visible through a viewing window when the boiler is idle, you have a pilot light.
If you have confirmed your boiler has a pilot light and it keeps going out, read on. If you have a newer boiler that is not firing at all, our guide on boiler not working in London covers electronic ignition faults in detail.
Why Your Pilot Light Keeps Going Out
When a boiler pilot light keeps going out repeatedly, there are four main culprits. Understanding which one applies to your situation helps you explain the fault clearly to a Gas Safe engineer and avoid being quoted for work you do not need.
1. Faulty Thermocouple (Most Common)
The thermocouple is a small temperature sensor positioned in the pilot flame. When the flame is lit and hot, the thermocouple generates a tiny electrical current that signals the gas valve to stay open. If the thermocouple is worn, bent out of position, or has failed, it cannot produce enough current — the gas valve closes as a safety measure and the pilot light goes out.
This is the most common reason a boiler pilot light keeps going out in London homes. The thermocouple itself costs £10–£30 as a part, and a Gas Safe engineer can replace it in under an hour. Total cost including labour is typically £50–£100. It is a straightforward repair on most older boiler models.
2. Draughts and Poor Sealing
London's housing stock is among the oldest in the UK. Properties in areas like Southwark, Tower Hamlets, and Haringey frequently have original sash windows, uninsulated floors, and gaps around pipework that allow cold air to move freely. A pilot light burns with a very small, low-pressure flame — a persistent draught through a cupboard or utility area can be enough to extinguish it.
If the pilot light goes out primarily on cold or windy days, or after external doors are opened, a draught is likely contributing. The fix may be as simple as sealing gaps around the boiler cupboard, though a Gas Safe engineer should confirm there is adequate ventilation before any sealing work is done.
3. Dirty or Blocked Pilot Jet
The pilot jet is a tiny orifice through which gas flows to feed the pilot flame. Over years of use, dust, debris, and combustion residues can partially block the jet, producing a weak flame that is easily extinguished. A weak, yellow, or flickering pilot flame — rather than a steady blue one — points toward a dirty jet.
Cleaning or replacing a pilot jet is a job for a Gas Safe engineer. It requires isolating the gas supply and disassembling part of the burner assembly.
4. Faulty Gas Valve
The gas valve controls the flow of gas to both the pilot and the main burner. A valve that is failing to hold open the pilot gas circuit will cause repeated pilot light dropout. Gas valve faults are less common than thermocouple failures but more expensive to repair. On a boiler that is already 20 or more years old, a faulty gas valve often makes replacement more cost-effective than repair.
| Cause | Likelihood | Typical London Repair Cost | DIY Safe? |
|---|---|---|---|
| Faulty thermocouple | Most common | £150–£220 (part + labour) | No |
| Draught extinguishing flame | Common in older properties | £80–£150 (diagnosis + draught sealing) | Partial (sealing only) |
| Dirty or blocked pilot jet | Moderate | £100–£180 (clean or replace) | No |
| Faulty gas valve | Less common | £250–£450 (or consider replacement) | No |
Repair Costs and What to Expect in London
London gas engineer call-out fees are typically higher than the national average. Most engineers in inner-London boroughs charge £60–£90 as a call-out fee before any work begins. At Trusted Plumber London, our £60 call-out fee is waived on jobs that run over two hours — which covers the majority of boiler pilot light repairs.
When an engineer visits, they will relight the pilot light (if it is out), observe its behaviour, and check the thermocouple output with a multimeter. A reading below around 10–15 millivolts typically confirms thermocouple failure. They will also inspect the flame colour and strength to assess the pilot jet, and check for any gas valve faults using manufacturer diagnostics where available.
If your boiler is more than 15–20 years old, discuss replacement with your engineer rather than committing to expensive repairs. Modern A-rated condensing boilers are significantly more efficient than older models, and the savings on gas bills — particularly relevant given London's high energy costs — can offset installation costs over a few years. Our boiler repair London team can advise on whether repair or replacement makes financial sense for your specific model and situation.
It is also worth checking whether your boiler has an up-to-date gas safety certificate. Landlords in London are legally required to hold a valid CP12 certificate, and even as a homeowner, an annual gas safety check is strongly recommended. Read our guide to gas safety certificates in London for more detail on what is required and what the inspection covers.
When to Call a Gas Safe Engineer
You should call a Gas Safe registered engineer if your boiler pilot light keeps going out after more than one relight attempt in a 24-hour period. There is no safe DIY repair available — the components involved are all part of the gas train, and working on them without registration is both illegal and dangerous.
The signs that make an urgent call necessary include: a smell of gas anywhere near the boiler, a pilot flame that burns orange or yellow rather than blue, a boiler that makes unusual sounds when the pilot is lit, or a pilot light that goes out immediately on release of the reset button (which indicates a thermocouple that has completely failed rather than one that is just degrading). In winter, also check whether the boiler flue is blocked or frozen — this can cause similar lockout symptoms. See our guide to a blocked or frozen boiler flue if the fault worsens during cold weather.
If you have recently had a drop in boiler pressure alongside pilot light problems, the two issues may be connected — particularly if you have an older combination boiler. Our article on boiler losing pressure in London covers the most common pressure-related faults and how they interact with ignition and pilot systems.
All Trusted Plumber London Gas Safe engineers are Checkatrade-listed and carry parts for the most common thermocouple sizes across older Baxi, Potterton, Ideal, and Glowworm boiler models. We cover all London boroughs and aim to attend within four hours on non-emergency calls, same day on urgent heating faults. If your boiler is also shutting itself off repeatedly alongside the pilot light problem, the two faults are likely related — mention both symptoms when you call.
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📞 Call 020 7870 3200Frequently Asked Questions
Why does my boiler pilot light keep going out?
The most common cause is a worn or faulty thermocouple — the small sensor that detects whether the pilot flame is lit and signals the gas valve to stay open. If the thermocouple fails, it cuts off the gas supply and the flame dies. Other causes include draughts blowing out the flame, a blocked or dirty pilot jet, or a failing gas valve. A Gas Safe engineer can diagnose the fault in a single visit.
Can I relight the pilot light myself?
Yes — relighting a pilot light using the reset procedure on your boiler is safe and is described in your boiler manual. However, if the pilot light keeps going out after relighting, you have an underlying fault that requires a Gas Safe registered engineer to diagnose and repair. Never attempt to adjust or replace gas components yourself.
How much does it cost to fix a pilot light that keeps going out in London?
A thermocouple replacement — the most common fix — typically costs £50–£100 for the part plus a call-out and labour fee. In London, expect to pay £150–£220 in total depending on the engineer and your borough. A blocked pilot jet is cheaper to fix; a faulty gas valve is more expensive and may make boiler replacement more cost-effective on an older unit.