On an older boiler with a permanent pilot light, the pilot going out is almost always a worn thermocouple, a dirty pilot jet or a draught, and a thermocouple is one of the cheapest parts on a boiler. On a modern boiler, which has no pilot light and ignites electronically, the same symptom shows as the boiler firing and then cutting out, or locking out on ignition with a flame loss code. The causes there are a dirty or failing flame sensor, an ignition electrode, a frozen or blocked condensate pipe, a gas supply problem or a flue fault. Both types are worth a couple of safe checks and then an engineer.
This guide covers the two generations of boiler separately, because the fixes are different, and explains which symptoms mean it is time to stop resetting and pick up the phone.
Older boilers: the pilot light keeps going out
Thermocouple
The thermocouple sits in the pilot flame and generates a tiny voltage that holds the gas valve open. As it ages the voltage falls, the valve closes and the pilot goes out, typically a few seconds after you release the ignition button. Replacing the thermocouple is a short job for a Gas Safe engineer and is the fix in most cases.
Dirty pilot jet
A pilot flame that is small, yellow or lazy rather than a crisp blue point is burning through a partly blocked jet and may not heat the thermocouple properly. The engineer cleans or replaces the jet.
Draughts
Open-flued boilers rely on room air, and a strong draught from a door, a vent or a windy flue can blow the pilot out. A one-off on a stormy night is nothing; a regular pattern needs the flue and ventilation checked.
Gas valve or supply
If the thermocouple and jet are fine, a failing gas valve or a supply pressure problem is next. Both are engineer-only.
Modern boilers: firing then cutting out, or flame loss codes
Flame sensor or ignition electrode
Modern boilers light with a spark and confirm the flame with a sensing electrode. If the sensor is dirty or worn, the boiler cannot see the flame it just lit, assumes ignition failed and shuts the gas off, then tries again and eventually locks out. Cleaning or replacing the electrodes is a routine repair.
Condensate pipe
A blocked or frozen condensate pipe backs water up into the boiler, the flame is disturbed or the boiler shuts down to protect itself, and the code is often an ignition or flame loss code. In winter this is the first thing to check. Our guide to a frozen condensate pipe explains how to thaw it.
Gas supply
Low gas pressure, a partly closed valve or air in the pipe after work on the supply all cause the flame to be weak and lost. Check other gas appliances work normally.
Flue and fan
A blocked flue, a fan that is slowing or a faulty air pressure switch can all cause the boiler to light and then cut out. These are engineer-only and worth attending promptly.
Gas valve and board
A gas valve that is not opening fully, or a control board fault, produces intermittent flame loss. Diagnosed with the casing off.
What you can safely check
- Is the gas on, and do other gas appliances work?
- Is the condensate pipe outside frozen or blocked?
- Is the pressure above 1 bar?
- Is the boiler in a draughty spot, and has the weather been windy?
- Reset once and note the code. Our guide to how to reset a boiler explains why once is the limit.
When to call
If the boiler cuts out more than once, if there is any smell of gas, if the flame is yellow rather than blue, or if the pilot will not stay lit at all, call a Gas Safe engineer. Repeated attempts to relight an old boiler with a faulty thermocouple can let unburnt gas build up, and repeated resets on a modern boiler with a flue fault risk carbon monoxide. Our guide to the signs you need a new boiler covers the point at which an old pilot-light boiler is not worth another repair.
How DCI Boilers can help
DCI Boilers repairs flame loss and ignition faults on old and modern boilers across Cornwall and Devon, with Gas Safe registered engineers who carry thermocouples, electrodes and sensors for the common models and quote a fixed price before starting. Book boiler repairs, or for a cold house our emergency boiler call-out runs around the clock. If the boiler is an old pilot-light model, our gas boiler replacement survey is free.
Need help now? Call DCI Boilers on 07888 862 603 or get a free quote.
Frequently asked questions
Why does my boiler pilot light keep going out?
Most often a worn thermocouple. A dirty pilot jet or a draught are the next most likely. All are quick fixes for an engineer.
Can I relight my boiler’s pilot light myself?
Yes, following the instructions on the boiler. If it will not stay lit after two attempts, stop and call an engineer.
Why does my modern boiler fire then cut out?
Usually a flame sensing electrode, a condensate blockage or a gas supply issue. It shows as a flame loss or ignition code.
Is a boiler with a pilot light worth keeping?
It is almost certainly non-condensing and over twenty years old. A thermocouple is a cheap repair, but the boiler is costing you more in fuel than a modern one would.